I've beaten it (all endings). Gotta say, it was a bit of a disappointment to finish the true ending.
SPOILERS AHEAD
WARNING WARNING
SPOILERS AHEAD
Once I entered the last portal, I was hoping to fight the C-Consciouness (it takes a robot-ish form or something). But instead all I got was that crappy cut-scene.
Other than that, it was a pretty good game. I've heard people say its "Oblivion with guns", and I agree completely. Problem is that there just aren't enough guns, armors, and tools to make up for the lack of areas. I was expecting Farcry, and got Armored Core (small areas).
I've started modding it. Just adding in some new weapons and armor, but I want to open up more of the map, add in a town and some factions. Someone is trying to restore vehicles to the game, which will make it so much better. Moving from one area to another took most of my time, and with the low carrying weight trying to stash stuff and then take it to a trader later is a pain in the ass.
Ratings:
Graphics: C- (Original Half-Life style)
Gameplay: A- (Highly customizable weapons and tough enemies)
Cinematics: B- (Not enough of them)
Language: C (I don't speak Russian, but it adds some flavor to being in the Ukraine)
So on my scale this game rates a C+. So much more could have been done with it, your left to wonder "How do you spend more than 5 years on a project and end up with just this?". But overall, its probably the best FPS of 2007. Halo 3 can change that, but with all the hype it has to live up to, S.T.A.L.K.E.R. may end up winning a game of the year.