Prey is a game that includes some pretty great ideas with the gravitational element being the best. It is pretty mind-bending to change gravity and walk on walls and ceiling. Sometimes you get almost dizzy if you play the game for too long. Spirit walking is also an interesting idea but it gets rather boring after a while since almost all puzzles are solved that way.
The puzzles are mostly quite straightforward and the opposition is not very hard. Even the bosses are fairly easy and the fact that you never die but just go to a "shooting tent" and then quickly returns to the game makes it hard to care if you get hit by enemies or not. On the other hand you won't have to use quick save at all.
The graphics are quite nice and the engine runs as smooth and great as we know it from Doom 3 and Quake 4. The overall gameplay is not that interesting in my opinion and I prefer the 2 other games just mentioned. The main character Tommy is a jerk, there is way too much flying around in hoover crafts and there is too few enemies to kill in many parts of the game.
Anyway if you like 3D shooters and you love the Doom3 engine (Id Tech 4) then you may enjoy parts of the game. It's not a bad game. It's just not as great as it could have been.
[Official site] [Walk-Through]