Decent game... but, unfortunately suffered from its own hype.
I may be breaking with the ranks, here, but I think Halo's greatness lies more in its ability to sell Xbox's than its actual gameplay. I really enjoy the game, but does it really set the standard?
Games like Half-life 2 and Far Cry have done much more for the genre of FPS. In fact, I believe this is because of the sense of adventure in these games. They may fit into the genre of FPS/adventure games, if there was one.
Anyway, Killzone is pretty standard. And for the PS2, this is a good choice. I would rather have bland environments than choppy framerate, if that is the decision.