Killzone 2 really killed itself with all the unnecessary hype. Its a fun game though, with a couple flaws.
The story mode is very mediocre. It felt like a Call Of Duty 4 storyline....not very involved in the story, its very easy, and it ends quickly. Basically, it all gets VERY repetitive.
I will admit, though, that the toughest difficulty level is quite a challenge. Also, the graphics and animations are stunning....you will be in awe after every cutscene. Other than that, its stuff we have all seen before, shoot em up, cutscene, new gun, repeat.
HOWEVER, when you play online, you start to see why everyone loves it. With several clever and lengthy game modes, you will quickly find yourself using the amazingly designed maps to utilize your tactility and dominate, hoping to unlock a new gun, ribbon, medal, or trophy.
Online, imo, saved the game. Once you play it, you will not be able to stop, especially with the amazing custom soundtrack on top of the other features.
In my quick summary, I think you should buy Killzone 2, if you like first person shooters. Its nothing in the game that will turn you away, and you will get your moneys worth.