Somewhat controversially, I don't think it's the best game ever. It's good, but it's not all that, girlfriend. I was playing through it wondering if there was something I was missing that all the other reviewers knew.
It's a good first person shooter with some interesting things, and if I were to go back and replay it (which I'm not itching to do, but probably will sometime), I'd probably enjoy it more the second time, now that the plot is a bit clearer.
And the ending... don't get me started on the ending.
It was nice when the actual main plotline eventually showed up, about 9 chapters in. Maybe I'm too used to being spoon fed plot in carefully marked cutscenes, rather than being forced to "do things" in the game for no apparent reason.
Highlights: Dog. Robert Culp's excellent offhand voice acting of Breen (I only wanted to scream "I lost the instruction manual!" at him once or twice). The Gravity Gun (capitals used, wink wink) (not during most of the game). The bit while waiting for the cart from Father Grigoriy. The stink grenade. Most of the bits towards the end.
Lowlights: Tunnel, do something, tunnel, loading. AI wasn't all it was cracked up to be ("Hey! You shot me! In the head! Hey! You shot me again! Hey!"). Lack of actual coherent plot until late in the game (and then the ending is just a cop-out). Total linearity. Obvious physics puzzle setups with only one right answer.
Would I recommend it? Sure. Despite its flaws, there's enough fun to be had to justify the purchase price. If this had come out 18 months ago, I'd have been floored. As it is, I'm fairly impressed, but it's not getting my Game of the Year award at this stage: Far Cry is prettier, and has better AI. Doom 3 was prettier, and had a more satisfying ending (more frustrating gameplay though). Max Payne 2 had a better story, similar physics engine, but less interaction with the environment (who cares!? Bullet Time 2.0!). Prince of Persia had better puzzles. Star Wars KOTOR had better sound. GTA: Vice City had more to do. Halo 2 has a more compelling story.
Maybe I'll change my mind by Dec 31, for the inaugural EBTDF Game of the Year Award*, but maybe not.