My take on Fiora as I've played her a bunch now.
As some has said, she is a Master Yi with boobs to an extent. She lacks a real escape like Yi but makes up for it in lane or jungling with a better sustain in my opinion. She does lack Yi's versatility when it comes to being AP or AD based though. Fiora is only going to be AD based despite 1 skill having an AP ratio (her W riposte).
What Fiora has that Yi doesn't is an active defense in the form of a "physical" shield. Both have an AD steriod. Yi's ult gives him an attack speed + movespeed steriod that lasts for a set duration. Fiora does as well but different. Again attack speed boost for 3 seconds (even higher than yi's attack speed boost), but the move speed doesn't occur until she hits something. which isn't hard if you activate your attack speed boost right next to your target. However, this means it can't be used as an escape unlike Yi.
Her biggest asset is her Ult. If timed correctly, it makes you untargetable for the duration. It can also dish out a ton of damage over all.
This is where Fiora exceeds Yi in one aspect. Against champions she is capable of bigger burst damage. She also has a gap closer that makes hitting her with skillshot disables much harder compared to Yi when she decides to enter the fight. It also allows her to get to the back line in team fights MUCH easier than yi to take out the other team's carries. That's her real role late game. Getting to those squishies and destroying them. She can take any range AD carry 1v1 at just about any time. That is assuming comparative levels and equipment though as any champ has a hard time killing another champ they are far behind.
I also think most people don't know how to build her right. She starts out with a high AD base and attack speed base. She starts out with AD steroid skills (her passive on her W) and an attack speed steroid. So the ONLY thing she really needs to start dishing out massive damage early on is crit % and crit damage. Here is how I build her and have totally demolished just about anyone top.
Runes:
Crit % Quints and Reds
Flat Armor Yellows
Mr/lvl Blues
This starts you with 14% crit chance. It's pretty significant and a couple crits and you are winning lane very early.
For Masteries I go.
21/9/0
Maximize your offense while having a little extra defense.
For items I typically start Dorans Blade. The extra health, AD, and life steal are welcome for her. With the lifesteal from the Doran's, Masteries, and her Passive you'll have very good sustain top.
The next part is knowing which skills to prioritize. I get a single point in Q and leave it there. I maximize that skills last. It is a great tool for closing the gap and to start harassing with. However, the steroids need to be built first and faster. I then start working on W and E after that. These are the skills needed to own people 1v1 and early. Prodigious use of her W is a must. Make sure you DO NOT harass during a cannon minion wave without taking out the cannon minion first. The cannon minion is considered a "large" minion and thus any attacks from it will trigger the riposte. Trust me you do not want this happening right before a Nasus fires off his Siphon Strike for a huge hit. I rather parry the Siphon Strike that may take a large chunk of my health instead of the cannon minion that will only tickle.
Her parry is what allows her to safely harass anyone in top lane just about. Just use the Q in to gap close while activating W and E at the same time. You'll get a few hits on them and then Q out to a minion and back off. You'll do WAY more damage to them than they will do to you. The only one that you won't be able to do this to is a Yorick with blue buff or enough mana regen. Not that I've had trouble in lane against a Yorick, just that once he reaches a certain point it's hard to kill him or harass him back.
As for items, you need to prioritize Crit % along with some defense. You really don't need attack speed with your steroid, but it doesn't hurt. Also, most crit % items come with attack speed usually. For items I typically build the following.
1) Dorans Blade
2) Boots
3) Avarice Blade (crit % and gold per 10 seconds)
4) Zeal
5) Merc Treads
6) Phage
7) Atmas
Do it in that order for the most part. With this build at this point you should be at 424 movespeed from zeal + boots. I prioritize zeal first over merc treads for the move speed. Merc treads will jump you to 390 move speed while zeal with tier 1 boots gives you 400 move speed. Phage is needed for more AD, built in slow, and HP buffer. With Avarice (12% crit chance), zeal (10% crit), atmas (18% crit), and runes (14% crit chance) you'll have a good high crit chance percentage at 54% crit chance. The Atmas will give you the armor needed to go toe to toe late game and survive team fights when you dive after squishies. When combined with the flat armor runes and armor from masteries it should net you 45 + 13 + 6 for 64 armor on top of what her armor scaling per level is. It should put you around 130ish armor around level 13 if memory serves me right. Same thing for MR. Merc treads + runes + masteries + scaling should net you around 110 MR at 13 as well. Not super tanky, but pretty darn good with the health from Phage and Doran's blade.
Going into late game this is what I upgrade and go for item wise
1) Zeal -> Phantom Dancer
2) Phage -> Frozen Mallet
3) Sell off Dorans Blade. Buy Vamp Scepter -> Blood Thirster
This should put your health around 2.8K easy.
The last item I deal with is Avarice Blade. It can be used for an upgrade to Ghost Blade. I find the attack speed I get from it to be worthless though as Fiora will hit 2.497 attack speed (cap is 2.5) with her E active and combined with Phantom Dancer. The movespeed bonus can be used as an escape, but I rarely need it as I have 443 movespeed with Phantom Dancer. The crit% is nice as it puts me at 82% crit chance at this point which gives me damn near a crit every hit. At this point I need to decide if I need more defense or offense. For offense I have two options to go with really. Infinity Edge, Last Whisperer, or another Blood Thirster. The Infinity Edge puts me at 95% crit chance with 50% more crit damage. You will crit every damn hit for massive damage every hit. Even enemies with decent armor values will still feel a bit of sting when you land an attack as your AD should be around 420ish at this point. So a crit with land for 1100+ every hit without before armor reduction. Those enemies with exceptionally high armor values 240+ won't be hurting as bad, but your attacks won't be tickles against them either. Squishies will go DOWN though almost instantly with your E active as you are doing about 2.5 attacks per second. Speaking of which, any time you kill an enemy champ, you will refresh your E to keep your attack speed active again. Just like Master Yi in that regards. The other offensive items Last Whisper is a no brainer if the entire enemy team is stacking armor values pretty high. Blood Thirster is nice if you are dealing with a bunch of other good duelists alot such as Master Yi, Olaf, and Riven. If a team is stacked with those I found having two Bloodthirsters makes you nigh unkillable by them in 1v1 scenarios. Still the second BT is much more situational at this point than either IE or LW.
For defensive items there are a couple to consider. Guardian Angel is great when the actual angel bit is up. It deters people from making you the focus target first in team fights as they know they have to kill you twice over while your team is beating on them while at the second time being quite a bit tankier over all with higher armor and MR. By level 18 the Ga will give you an armor value at 220 with an Mr value at around 160. Combine that with 2.8K health and you are pretty damn tough to kill at this point. Another item to consider it Banshee's Veil. If the enemy team is full of disables and AP champs then this might be the wiser choice over GA. You'll boost your MR up to 180ish or so while giving yourself more health. This will put you over 3K health while giving a few more AD points from atmas. Last but not least is Aegis of Legion. If you aren't being specifically focused, and the enemy team is compromised of a lot of AoE, then this may be the better choice. A bit more armor, MR, and health for you as well as some additional AD. It's also nice that is shares some of that with the rest of your team.
Basically the build I do maximizes her potential I've found. Building up more AD and/or attack speed on her is stupid. Especially early on. You don't need it. Her W maxed out is 35 additional AD all the time. That's basically a free BF sword. Her E maxed out is 120% attack speed for 3 seconds. Again that's HUGE! you don't need much more as that will always put you above 1.8 attack speed at that point. Offensively wise the main thing she is lacking is crit % so that's what I build for. Beyond that she needs a little bit of defense to stay in the fight.
Speaking of which she makes an excellent top or jungle. What makes her one of the better tops in my opinion? From level 1 she out dps's everyone. Thus you need to go after your enemy counter part early and hard. Be relentless in harassing. NO ONE can trade blows with Fiora early. Especially once you hit level 3 and have Q, W, and E all usable. Your job at top is to hit them hard and constantly. They will back off or they will die. Speaking of which, there is a downside to this. By being that aggressive early you will most likely draw the enemy jungler to top. More so if you are starting on the blue team as the purple jungler's red buff is right next to top. Just be wary that the enemy jungler will most likely camp you top if they want to let their top have some chance at doing anything there early. This is fine if the rest of your lanes aren't losing. As long as you keep that jungler top then you draw that jungler away from being able to gank other lanes. If the enemy jungler doesn't come top to camp you for ganks then you will kill that enemy top or keep sending them back to base. You'll win that lane easy. I don't care what opponent Fiora is facing top. None of them have the sustain, DPS, or tankiness to deal with her in the first 3 levels. Even ranged champs can't deal with her since she starts with a double gap closer.
As far as jungling goes, she makes a good jungler as well. Q for gap closing on ganks. Her Parry works against the big minion at each spawn to remove incoming damage while giving the minion damage. Her passive for sustain, and her E to finish monsters while giving her a movespeed boost to get to the next camp. Her downside as jungler is a lack of CC to hold targets during a gank in place.
During mid game she is very strong as an assassin. If you are top lane, roaming down to mid lane to help on a gank is very easy to do. Her Ult allows her to tower dive with impunity as the tower can not target you to hit you while in your ult. Too bad she can't parry a tower hit though. Still if you get a few hits with E to finish a target after using your ult on them then you'll get a move speed bonus to escape the tower without taking too many hits once your ult is over. Her ult and damage output pretty much guarantee her a kill in 2v2 or 3v3 situations while still giving her a chance to make it out alive. Press your advantage with her as you can as long as you have levels and items over your foes.
Late game and big team fights she doesn't shine as much. You'll find one of two things happen. You are either massively targeted first, or not. If not it's simple to just rape the other team. Just target one of the squishie carries and kill them fast. Move on to the next squishie target after that. Ignoring a Fiora is dangers due to her damage output. However, you'll frequently find that you will be the initial focus in team fights and late game. The other team is most likely scared shitless from you if you've been playing as well as I described above. They will drop every CC, summoner, and ult on top of your head. Hence why you need to build some tanky items with her. At this point you become the damage soak and hope the rest of your team can finish up their team once they've blown everything they have to take you down. I find late games I end up dying much more often as Fiora, but it is always a good trade as I die for 3 or 4 of the enemy team deaths.
Which brings me at last to summoner spells and what to take. This is a hard one. Late game either cleanse or heal are going to be more beneficial if you are getting to be the focus during team fights constantly. Especially if the other team has plenty of CC. Exhaust and Ignite are going to give you way more kills early on to hopefully give you a huge advantage early to mid game though. Flash is a must unless you are jungling. At which point you can get away with smite and exhaust without having flash.
I find overall Fiora to be an excellent champ. She trades blows better than any other champ early on. She can be devastating mid game to get her a massive advantage to press earlier than other champs. She is still useful late games and can even burst down towers pretty good. I find that her Parry works on quite a few abilities besides plain auto attacks. Of course any ability that does physical damage while applying on hit effects it wil parry. Such as Siphon Strike by Nasus or Parrrley from Gangplank. It will also parry some of the pure physical attacks single shots such as Caitlyn's Ult or Corki's Rocket. I'm not sure if that is suppose to work that way, but it currently does. The great thing is it doesn't have a particle effect that is noticeable to let people know you have parry up. So they may waste an attack without realizing it. While her Parry isn't quite a good as a real spell shield, it certainly does help in mitigating quite a bit of damage over the course of the game against her.