I beat the game today with an 87% completion rate in 17.5 hours. I think I missed three optional missions so if you do everything the game will probably take between 16 and 18 hours. It's a very good game but falls short of being great.
For me the biggest plus DS3 had going for it was it's length. It's a lot longer than the previous two games and the levels are a lot larger. It's a vast improvement over DS2 which can be beaten fairly quickly (under 7 hours) especially once you've played it before.
The biggest minus was the weaponplay. The guns were a lot more fun to use in the previous two titles. Guns like the plasma cutter, line gun and ripper were a ton of fun to play with. Now the plasma cutter is useless, the line gun is nerfed and the ripper isn't that effective. The enemies are now a lot quicker so there's not a lot of time for "strategic dismemberment" as in the previous ones. DS2 had quicker enemies that made stasis necessary but it still was slow enough to allow for strategic dismemberment. DS3 is basically aim for the limbs rather than carefully dissect your enemies.
I should note that I'm playing with a controller so the feel of the guns includes the vibration. In the previous two the guns felt right. The line gun felt like you shot a large beam, the ripper felt like you were tearing into necromorph flesh. They were also a lot more effective. Fully upgraded the line gun would dismember both legs with a precise shot. In DS3 it sometimes took three precise shots to dismember legs. It's supposed to be a slow yet powerful weapon but in DS3 it often ended up just being slow with mediocre damage. With the ripper in the previous two you could hold necromorphs at bay by backing up into a corner and tearing into their limbs with the saw. Now you can't do that since the necromorphs will continue to come at you when you're tearing into their limbs. It's still somewhat effective for hoards but isn't a weapon you'd want to use against regular necromorphs.
Overall, the gunplay is decent but not as much fun as the previous two. Hopefully they'll tweak it and the necromorphs behaviour in the next game and get the balance right.
As for some nitpicking, first I want to comment on the addition of human enemies. The gunplay doesn't work well against human opponents and the cover system isn't very good. The encounters against humans aren't overly hard, they just don't fit into the gameplay of a dead space game.
Although the story picks up at the end for about 2/3 of the game it's badly executed. I don't know how it is with your co-op partner there but for soloing the story is mediocre for at least half of the game. The overall plot is good, it's the execution that's lacking.
DS3 didn't turn into a cod action fest like some feared but there's little to no of the horror you'd expect for this type of game. There's no tension/creepiness like you felt in the first and nothing disquieting like the children necromorphs of the second. I'd like to see a return to that.
While the suits are one of the iconic images of the DS games here they're just cosmetic. When you upgrade your rig it applies to all the suits so it doesn't matter which one you chose. I wish there had been some difference in the suits like in DS2 where diffenet suits gave you slight bonuses such as statis recharge time or weapon damage.
I've already mentioned the weapon configuration system before and don't really have anything to add. Thematically it fits with Isaac being an engineer and the only thing I can see being a downside about it is that it takes a while to learn. It's a welcome change but I'd like it a lot more if the gunplay was better.
Overall, I think that DS3 is a very good yet slightly flawed game. It'll be one of the better games to come out this year but it's flaws keep it from being in the potential game of the year class. I don't regret paying near msrp for it but for those of you on the fence there's no problem with waiting for a price drop. Right now I plan on going back and trying the co-op.