The online play is sluggish. There is a serious lag between the time you make contact with the controller to the time your player responds. It is so bad that it makes horrible users better, and it makes great users worse. Target acquisition, using either the iron sights or the green dot sight, is almost impossible.
Don’t get me wrong, the campaign gameplay is mind-blowing. -It’s great. But it's obvious that the online mode was seriously neglected. It’s so bad that it makes Resistance 2 multiplayer look like a work of genius.
If you are used to CoD4 and World at War, then there is no way you are going to like the way this game feels. Now, in terms of graphics and overall coolness, Killzone 2 has any game beat. But no amount of Next-Gen graphics can substitute for gameplay. Trust me, I hate Call of Duty: World at War... but at least (when the servers work) you can actually shoot someone!
I pre-orded Killzone 2 almost two years ago. And to be honest, I would rather play Killzone 1 on PS2.