Killzone 2 can basically be summed up with my title. It has an amazing single player campaign but also has a multiplayer that is a little hard to jump into but is rewarding in the long run.
First off, this if the first PS3 game that I have played and I am quite happy with my first choice. When you start the game there is a pretty intense introduction sequence before the start menu that already gives you an idea of what you're going to expect in the game. Basically, you are a soldier of the ISA which is taking over the planet of Helghan. Just like most first person shooters, you are given instructions by you're commanding officer to go here, shoot that, blow up this, etc., but it never really gets old. The level design, while very guided, is great and action is always waiting right around the corner.
The weapons in the game are very fun to shoot and fit the usual line up. There are assault rifles, SMG's, a shotgun and a couple guns that you wouldn't always expect like a bolt gun and an electrical weapon blaster thingy.
The story, while not the greatest, is engaging and enjoyable. I came attached to a couple of your squad mates in the game because they are quite hilarious. The cutscenes and graphics are beautiful and most of the time the cutscene will fade and you'll be back in control without a loading screen.
The multiplayer, once everything is unlocked reminds me a little of Team Fortress 2 with the class system. One thing that is different about Killzone 2's gametypes if that they are all in one. About every five minutes the gametype will switch in game. You'll be trying to assassinate an enemy member one moment and the be told to guard to strategic point on the map the next. This constant change on the battlefield is a lot of fun and forces you to stay on your toes.
Killzone 2 is a must have for ever PS3 owner. The single player is great and the online is just as good. The only complaint I have is that I can't use my 360 controller because I find it more comfortable. You can't have everything you want now can you? ;)