The other reviewers touched on much of the story, and basics of the game, so I wont get into that. I'll just add my comments from playing and beating it.
All in all it was pretty good, the story was decent, although I thought it could be a bit more in depth, the graphics are stellar, and alot of the new gameplay aspects are cool. Such as the portals, wall walking, and the things you shoot to flip the room (although those can get old fast)
The AI was okay, nothing great tho, but not super easy, but you wont be crying to throw on "god" mode any time during the game.
Lastly, I also think this was one of the easiest FPS' of all time, the puzzles were very simple (which is good, since this IS an FPS), the game feels very linear, and a lot of it gets repeatitve as hell.
+ Good Graphics