My first FPS game on the PS3 was Killzone and I an now a fan.
Something i found weird about the game's campaign is that its really short. I bought it and i had finished it the next day. Maybe the major reason could have been the normal difficulty i choose to play in. The cut scenes were too long and boring.
The co-op campaign had a little lag here and there but it too was amazing. The online multi-player is amazing and i especially like the guerilla warfare type games.
Overall i think it is a high speed, blood thirsty game which we can always play whenever we feel a little trigger happy.
Hope to see the next game even better.