Prey is a good game, with a few interesting ideas aswell as a few problems.
The gameplay is fairly average, although the idea of portals are pretty interesting, since you can shoot through the portals, which is sort of new to this game. The whole gravity switch idea is also a nice idea since they're sometimes used in puzzles: for example to may need to shoot something which will flip the room upside down or on its side. The gravity walkways also work well and are sometimes used in puzzles as is the "spirit-walk" ability, which is when you come out of your body in your "spirit form" and are equipped with your "spirit-bow". There are also some puzzles that don't used the gravity stuff for example: pushing some sort of bomb in to destroy some organic stuff leading to a door. None of these puzzles are particularly difficult or interesting, for that matter, but they do add some surfistication to the game.
This game is essentially a shoot 'em up. Because of the deathwalk ability you can be as "Gung-ho" as you like. The enemy AI is certainly nothing special, the enemies will occasionally duck into cover or lob a grenade your way, but as I said nothing special. The campaign is about 10 hours long with 24 levels, although 3 aren't really levels. Long, easy and fun.
The online is fun, but is often laggy, which can be awfully frustrating and annoying. The mode is still worth checking out though, simply for a multitude of weapons and a few good maps and of course the achievements.
The graphics are pretty good. Prey does the whole mechanics and metal fused together thing really well with good looking environments and everything looking really sharp. The weapon design is nothing amazing, the machine gun (with the grenade launcher) is the best looking in my opinion. The enemies look ok, look pretty alien and there are some interesting designs. I suppose they could have been a bit better, but there are no rough edges, although the frame-rate occasionally drops a bit in some vehicle sequences and when there's lots of enemies on screen, but it's managable.
The Sounds is good. All the weapons sounds different and quite realistic. However, after a while the enemies sounds stupid as they speak in english and sound like someone burping words. The voice acting is pretty mediocre, your girlfriend, Jen, sounds really bad, with her whiney, nagging voice. Tommy (you) and his grandad sound pretty good and the bosses also sound good and alien like.
As I said the game has a good, long campaign, and a few online modes which are worth checking out. The achievements are really well balanced between the single and multiplayer. Basically, you get most for completing the campaign and some for getting certain amount of kills, amount of kills with certain weapons e.t.c.
Overall Prey is a stardard, fun to play, FPS, which is worth picking up. It's nothings particularly special, but worth it, with a lengthy campaign and good online modes.