Awesome, but nothing spectacular
The game is about a war in outer space between two factions, the bad guys and the good guys, you are the good guy. The story wasn't attractive at all so to be honest I didn't pay attention why they were fighting in the first place. But I didn't care much for lack of story since the action keeps you going.
The game relays, to a point, on the cover system. You need to keep the L2 button down as long as you need a cover which can be a pain in the neck when you are in a long fight, but you will need to keep moving and changing covers most of the time so it is not a big deal.
Effects in general are beautiful. The Blood shooting out of enemies, hearing them scream when they get shot, and watching your opponents squirm in pain when they get a non-fatal shot is just so much fun. What even makes it more fun is the variety of guns. They all look realistic, have different functions, and amazing sound effects.
Multiplayer is awesome. I didn't get so much time to try it, but it seems very comprable to what Call Of Duty 4 had to offer. When you score a kill or a kill assist you get experience points, once you hit a certain amount of points you unlock new weapons. It just keeps you demanding for more.
In general, killzone 2 is awesome. Good single player and perfect multiplayer are enough reason to get the game as soon as possible.
Hope that help :)