I got the game yesterday, and was playing it on and off all afternoon/night (probably put 2-3 hours total into it, I left it paused quite a bit). I just got to chapter 5.
It's visually impressive, but technically a little rough around the edges. Light that should be static shimmers sometimes, there's artifacting on some edges (like a thick white line of pixels on the edge of the main character's hair, as if the alpha sections of the texture weren't cleaned up well). The environments look great though, with huge towering buildings and ruins. It could have used a little more visual polish to clean things up, but the basic layout and deisgn is great.
The gameplay seems pretty simple so far, with some very linear and forgiving climbing (you can't jump to your death, it will only let you jump if there's a climb point there). The combat is pretty simple, at least at first (upgrades give new moves and such), but is still sort of fun. I say sort of because of the camera, which is the only real gripe I have with the game. Incombat, the camera likes to get really close, and I often find it between an enemy and my character. That makes it a little difficult to hit the enemy. Then they throw in enemies that charge up a ranged attack, and you can't tell they're doing it unless you mess with the camera in combat, which requires the thumb you're hitting the attack buttons with.
The chatter and small interactions between the characters is great. Actually, all the interactions are. It adds a sense of depth and emotion that makes me think the game would feel stale and boring if it were just one guy running around these places alone. They (so far) do a good job of taking advantage of having two characters alway there, and making you worry about protecting the person you're escorting, but I've yet to feel like it's "That Shitty Escort Mission - The Game".
I really don't want to stop playing, but I need some sleep. It's not "holy shit" amazing, but it is a lot of fun and I feel compelled to keep going.