Killzone 2 is the best of the series imo. I've had that since before I got my first PS3 console This is just a spare...I might send it over to my sister's house actually.
I've got all the sp trophies but haven't got the unfortunately. Hardest difficulty sp is pretty tough, tougher than CoD WaW imo but it isn't impossible.
Online can be difficult (I have played it a fair bit though even with my lag) plus servers don't record your stats properly now...so it's kinda broken.