I beat the game over the long weekend. My immediate impression is that it did some great things, but that it was also flawed enough that it wasn't quite amazing. Played on moderate, with everything like the hint and marking system turned off. Took me about 13-14 hours I think. I forgot to check my final time before I started a crushing playthrough.
I liked the new climbing mechanics, though I felt the piton was underutilized until the last couple climbing sections. I loved the rope, and the freeform climbing. Sure, it's still not challenging gameplay, but I found it really relaxing.
The art direction was stellar. I'm not one to explore a world for the sake of exploration, but I definitely did that here. There was something that just drew me into the world, unlike anything I've played before. Not even their previous effort, TLOU.
Gunplay was great. Loved the reticle system, and the guns felt satisfying to use. The stealth mechanics were great too. The open arenas were awesome, and it was fun utilizing stealth until the shit hit the fan.
Story was decent enough, but I loved the ending and epilogue. It was a fitting end to Nathan's Story.
I also enjoyed the last boss fight.
As for negatives, I felt the pacing was off. The beginning was pretty slow, and the boat chase felt like a weird place to start the game. It didn't grab me the same way Uncharted 2 did with waking up in the train and wanting to know how he got there. It was just, oh, this is kind of random, I'm driving a boat, ramming other boats.
I would have also liked to have seen more combat arenas. They were a ton of fun, but felt like there were too few. On the opposite side, a few of the forced combat sections were annoying, particularly the ones that were in the interior of the pirate ships. It's annoying being shot at from 5 different directions at once in a small enclosed area, at least until you figure out where the shotgunner spawns from and which was he goes.
Overall, I loved the game, and had a blast with it. It just didn't quite strike the same cord that my favorites have.