Killzone 2 has a pretty short campaign, but makes up for that with amazing graphics and an active and fun online mode.
In Warzone (online), you can make squads during battles. Squads allow you to keep in touch with three (max.) different people. And if you die and you're squad leader is still alive, you can respawn right next to them. That will really come in handy if he is about to be killed. You could save his life.
And all the different classes you can unlock are cool, too. I have only unlocked two new ones, though (Medic and Engineer). With the medic class, you can revive your fallen teammates, but you're only allowed to use assault rifles. So that means no shotguns, snipers, rocket launchers, or heavy machine guns. And when you're an engineer, you can place turrets to shoot any enemy that walks by it. But that turret can be destroyed any time by an enemy that shoots at it enough or throws enough greandes at it. And the only weapon you can use as an engineer is a shotgun. So it's best to be an engineer in small levels.
This game is probably the best FPS for the PS3. And even if you aren't a fan of shooters, you should still get this game. There's a large chance that you will love it.