Well this is only the second game I've ever purchased on release day (the other being GTA 4) but I simply couldn't wait to my hands on it and I'm glad I didn't wait. As soon as the first cinematic came on and I was dropped onto the battlefield, I knew I was in for one the best gaming experiences ever. Killzone 2 is just everything you could really want in a shooter. The graphics are stunning, gameplay is excellent, AI is actually fairly intelligent, and it's got some of the best sound effects I've ever heard. The Helghast are also some of the best enemies ever. They actually work together with eachother to try and take you down most are smart enough to take cover and constantly move to try to flank you rather than just stand in the open firing blindly. And lets face it. They look awesome and sound pretty intimidating. Well at least until they get shot in which case some of them scream like little girls. Just be prepared to die quite a bit because this game is also very difficult.
I highly recommend Killzone 2 to really anyone who has PS3. You'd have to be crazy to miss out on this game.