Although I wasn't expecting much after Killzone for the PS2, Killzone for the PSP is more than satisfactory.
At least the gameplay isn't as stupid as the story. This game doesn't mess around and will punish you for your errors. You can even try it on easy mode and find that the enemies will shoot you to pieces if you don't use strategy. Finding cover, using suppressing fire and grenades is a must. It's like a simplified 'Brothers in Arms'; you will at most have one AI-controlled ally to order around. The one thing that mars the gameplay somewhat, despite its general solidity, is the aiming system. It's a kind of auto-aim that requires you to point in the general direction of the enemy. This can prove to be finicky, even quite annoying at times.
The level design is quite derivative. There's very little that's new here, and if you don't generally enjoy this genre Killzone is probably too frustrating or too unrewarding to bother playing. Although the game is fairly generic, there are a few instances of good level design. In particular, the last level is excellent.
Have no illusions, Killzone isn't superb ,but it is quite good. In terms of action games, I found Syphon Filter: Dark Mirror to be far better designed in gameplay, presentation, and story and would recommend it instead of this. However, given how relatively few standout games there are for the PSP, it's a worthy addition to any library.