Serious Sam 2 is one of the funniest and over the top first person shoot-em-up I have ever played (and I've been playing since the original doom). Wonderful sense of humour in the game, and in my opinion has the essence of FPS's at it's core. Absolutely loads of levels (the game seemed endless when I first played it! :-) ) From giant robotic spiders to demons with ridiculously big guns.
What is the absolutely icing on the cake with this is that it works with the nvidia 3d vision glasses (you have to have the appropriate monitor and glasses though to get 3d). The 3d effects in this game are mind blowing using the 3d shutter glasses. The 3d isn't without it's occasional glitch, but the overall effect is a near perfect nvidia 3d vision experience.
This shouldn't be confused with the original serious sam games that didn't have the vocal sense of humour in the game and are missing that magical spark that make them great games (in my opinion). To be fair, the first serious sam didn't have any funny commentry but did have some good over the top baddies.
Loads of replay value in this (I keep replaying the whole game about once a year, and despite having many of the new games like AVP3 and COD MW2, I'm going through it again because of the 3D glasses.)
I seriously recommend looking at some video footage on youtube maybe to get an idea of the gameplay and if you don't mind some spoilers than maybe some of the cutscenes. 10/10. It deserves no less.
Oh, and I nearly forgot, I have this running on Windows 7 Ultimate 64bit no issues.