It's a shame, Killzone is a series i honestly WANT to like, but it just doesn't deliver. The concept is perfect, the graphics, setting, enemies, weapons, the hyper-realistic reloads, everything is in the right place to make a blockbuster shooter, but theres just something missing. Something about the gameplay just isn't fun to me, and yet i know the games SHOULD be great. I don't know if there is a designer somewhere crying himself to sleep because they butchered his vision of the perfect FPS, but i just can't get into it. My favourite KZ so far has been Liberation on the PSP, which kind of speaks for how i feel about the other games.
As for the coop, it's a good idea (a very LATE idea, i might add) but after watching some videos of it, wow, that's BAD.
I'm a big fan of coop games, Gears, Halo, Army of Two, to name but a few, which of course led me to go for the 360 rather than the PS3 despite my last two generation consoles being the PS1 and PS2. I got KZ on the PS2, and while the overall feel of the game was great, i just couldn't play past the dock level because i was so bored. I have been looking for an excuse to get a PS3 lately, and while i was hoping that excuse would be Killzone, it just doesn't look like it's going to deliver.