Killzone 3 is by far the best in the series! Watching gamplay on the computer doesn't compare to actually playing it.
The (story mode) is in my opinion good too especially if you already know what the story is about from recent past titles. Some say the don't like the story well pay attention to the text in the beginning and play the other killzone's
The (gameplay) is very smooth and are easy to get use to. (Multiplayer) is the best, well balanced and extremely addicting also rewarding!
Its been tweaked since the beta stopped and its great! it feels as if the thing we call quality is there and it is :-).
Overall: The Game is the best first person shooter i have ever played hands down. Its worth it. I'm happy i purchased Killzone 3 and being happy is what counts ;-)