This is not a game to pick up to have mindless fun. Because everything that can go wrong eventually will. Unfortunately not just in the game.
Metro 2033 is set in a postapocalyptic Russian cityscape where only the lucky(?) commuters in the famous Moscow underground survived the nuclear blasts - and are now tormented by the cruel conditions they have to face. The underground tunnels belong to surviving humans ranging to both ends of the moral spectrum and various mutant creatures, all trying to make the most of their life.
And life is harsh. Ammunition is so scarce it is used as currency. So you have to be very careful with your aim. Having to make every shot count may sound fun but the next time you get caught with an almost empty weapon between nasty mutants and bloodthirsty enemies you may long for a more generous game design.
The weapons are not many but they are well designed. Both the pre-war and the improvised ones offer more or less realistic mechanics and satisfying results.
Light and shadows play an important role and stealth is something you will be thinking quite a lot - especially if you are low on ammunition. The graphics of the environments are detailed and beautiful whereas the movements seem fluid and natural. The game designers aimed in increasing the immersion factor wherever they could (there is no HUD besides your cross-hairs, you have to hit T to take a look at your watch - very important when venturing into the irradiated cities); however, pop-up messages and stuttering take a big bite out of that.
Metro 2033 will inevitably be compared to S.T.A.L.K.E.R.. I found S.T.A.L.K.E.R. to be more of an RPG (hence its open sandbox nature) whereas METRO 2033 is more of a shooter with a predestined path to follow.
The PC I run this maxed out is a 2 years old system (WinXP SP3, P7 920 on MSI Eclipse with 3GB of RAM and an ASUS nVIDIA GTX480) and the game showed a proneness to stuttering, especially when enemies swarmed. I guess a future patch could take care of that but I would had preferred to receive a finished game and not one rough around its edges.
The retail version of Metro 2033 comes with Red Faction: Guerrilla as a bonus. It is not a bad game and, contrary to Metro, it offers more fun than immersion.
Finally, even he retail version of the game will require to be tied to a STEAM account. Yes that means ownership of your game will be stolen back and you will be allowed to play with it but not actually keep it. Whether this is acceptable to you or not you can now make an informed decision.
Tread with caution.