So after a slight upgrade to my PC (new SSD's, HDD and fans) I finally got to play a game in about a week. Played Leona and I must say, not impressed.
Like others have said, she's weak as shit (damage wise, even for a "tank"), needs mana regen items (money taken away from tank items) and, this was the one that pissed me off the most, I think she may have the WORST move speed in the game. Holy shit she's slow. I know she has a gap closer, but if you ever need to run away, don't bother because you ain't getting away with her slow ass.
I actually do alright with her. I use her as a counter pick to certain heroes in ranked. She is great for interrupting channeling ults with her double stuns. I do this for her build everytime and it works for me.
Regrowth Pendant + HP Pot
Complete Philo stone (takes care of almost all your mana problems and gets you some gold)
Boots
Heart Of Gold (add some HP and more gold which she needs cause her farming sucks)
Complete Merc Treads
Ruby + Cloth Armor + Null Cloak -> Aegis of the Legion
Phage (Hps and a chance for a slow and some AD)
Zeal (Attack speed, crit, and move speed)
Complete Tri Force
Complete Randuins
This nets me at level 18 around 3K hps, 160 AD, 190 base armor, 130 MR, and 420 movement. With the gap closer, a couple of well timed stuns, slows from tri force and randiuns, plus a decent movement speed will mean that no one gets away without flash. When I use Leona's W for her "AoE shield" it adds about 50 armor and MR if I remember. That puts you into a nice tanky place for team fights with that much HP and resistances.
Usually it is game over by this point but depending upon the circumstances of the game I'll pick up one of these items next:
Complete Shureliya's Reverie from Philo stone (which nets another 300 hps as well for more tankiness and speed!)
Thornmail versus AD team
Banshee's veil against MR team with lots of CC
FoN versus MR team with little CC
Her attacks are fairly weak. Her auto attack is nothing spectacular at all, but with Tri Force it does more than tickle at least. Her best asset is her ability to close the gap on the squishies in the back of a team fight and take them out of commission. The trick is to prioritize her stuns as much as possible. Her gap closer roots them for a second as well.
Leona's main source of damage in team fights come from her passive really. Her W popping is also decent as well. Her passive stacks damage on top of what your team mates do. So after Leona hits, then they need to hit. Leona's can not be passive in team fights as they only hamper their team and their damage output. Don't be afraid to press W for your light shield, tap E in the direction of an enemy you want to close the gap on, and then Q to stun then as soon as they move after the root wears off. On a squishie, if you have ANYONE on your team hitting them while you do those three abilities you WILL kill a squishie. Even on bruisers it only takes your Ult right after the Q stun wears off to finish of most mildly tanky bruiser characters. Just remember to lead the enemy with your ult so they get hit dead in the center of the ult for max effect.
Again the trick with Leona is she NEEDS someone working with her. He can cause pretty damn decent damage if you have someone who is on the ball by hitting their abilities on the same target as her.
Still the majority of the kills I get for her are early and mid game. Her damage is sill decent, but it really tapers off late game comparatively to how much more survivable enemy champs get late game. Still if you aren't getting a kill with her at level 3 or 4 in the laning phase then you are doing something wrong. Her ability to dominate the bottom lane, even against other early game bruisers like Jarvan, Garen, and Rumble make her great at slowing those characters down to the point they become non factors late game usually.