Too many issues hold Killzone 2 back from being a great game.
There's some great things about this game that make it fun to play. The graphics look great, the sound is perfect and the music is perfectly atmospheric, the gameplay is solid and VERY challenging (sometimes frustratingly so). And there were plenty of cool looking/sounding weapons that are a blast to use.
But it just isn't enough to make it a great game. The story is unoriginal and bland. Most of the environments are dark and the enemies all look similar (not to mention the fact that I couldn't tell my own teammates from the enemies much of the time). There are no really memorable boss fights...even the final "boss" fight isn't really a boss fight and not very climatic. Loading times will literally freeze the game momentarily to load the next section. You can only hold one weapon at a time making it a pain to run around looking for another gun and takes away the freedom to attack as you choose. And what's with the controls??? I can't tell you how many times I cursed having to press L3 to run and dying from not being able to get out of an area fast enough. And that stupid tacked on six-axis control...I kind of wish Sony never implemented that feature because I can't think of any games that really use it effectively instead of it being used as a tacked on gimmick that kills the flow of the gameplay.
Killzone 2 ultimately delivers a fun experience, but I would definitely rent it first before a purchase.