The framerate is so bad that all of this hardly matters. It makes Advent Rising seem smooth.
Killzone features quite possibly the most unplayable framerate I've encountered this gen. It truly comes close to matching N64's Goldeneye (though, thankfully, it isn't as bad as Perfect Dark...). It's just so damn low.
There was a solid base there, though. The actual gun wielding is suprisingly solid and fun, the atmosphere is absolutely killer, the level designs feature some inspired architecture, and the game has a good sense of weight to its character movement.
The engine just can't handle the vision, though. Outside of the framerate, there were so many scenes in the game where I could really taste what they were aiming for but found the end results disappointing. Something along the lines of the cherry trees in the park. The blooming sakura amidst the gloomy backdrop was a great design idea. That idea could have made for such a dramatic and beautiful scene...but it was held back by their engine. I don't believe it was impossible for the PS2 either.
I do have some hope for the sequel, though. We'll have to wait and see, though.