After 3 days of play, here are my first impressions:
Killzone 2 doesn't seem to offer anything new. The story has a lot of WOW but feels Starship Troopers cheesy. The single player game play is straightforward, but I think I will find myself playing it a few times over to get all the game trophies. While playing the single player campaign, you do feel like a single soldier in a MASSIVE war, but the feeling of being irrelevant isn't always a good thing.
Killzone 2 is all about borrowing ideas, combining the best of them and executing it flawlessly. The single player component borrows a cover system from the likes of Gears of War. I first saw the squad-based combat system in Call of Duty and more recently in Battlefield: Bad Company.
The multi player component borrows elements from many many games. I first saw the concept of being "mortally wounded" in Wolfenstien: Enemy Territory, Killzone uses that mechanic and it's implemented well. I love being able to walk up to a downed enemy soldier and shooting them in the face with a pistol while they struggle for their last breath.
The optional squad system is straight from Battlefield 2, and it's executed perfectly. The class system is also borrowed from the likes of Battlefield. The engineer and medic classes are borrowed from the likes Team Fortress 2.
The multi player game play is challenging and a hell of a lot of fun. Every few minutes you'll be changing your target to advance towards a new objective.
One of my favorite game modes is "Assassination", which I'm sure we've all seen before in one game or another. But this is a real lot of fun, the assassination target is just a regular guy and your side has to either protect or kill the target. I've been the target many a time and begging everyone to save your arse while you desperately fight to stay alive is hell fun!
Killzone's online service is near flawless, voice communication is crystal clear and lag is extremely rare.