I am trying to play a lot of Killzone 2, but The Watchmen and family life has been getting in the way. Regardless I have played some of the EXCELLENT single player campaign and have played enough multiplayer to gain the Sergeant rank.
The Multiplayer is amazing, it has a weighty feel and your character can only take a few bullets making them seem more human. The movement is more natural and turning is at human speeds (not the normal turn on a dime speeds of standard FPS games). The environments and models are amazing in multiplayer as well.
The game modes are truly fun, with an overarching mode called Warzone you basically have staged actions between the two factions. During the course of Warzone you will seamlessly switch between the modes such as Body Count (Team DM), Assassination (one person is the target and you kill or protect depending on the side) and search and destroy (plant C4 on targets or protect yours). When a mode is completed (you achieve/lose the objective) the game keeps running, if you were in a firefight it keeps going. The only time you hit a menu again is when the warzone is complete and all modes have occurred randomly.
It is a great system and really innovative. The only gripe is the class system is completely locked till you level up and leveling is a time consuming affair. All players start as soldiers with basic weapons and rewards are given as you gain rank. A little unforgiving, but enjoyable nonetheless.
Further enhancing the experience is the 'My Killzone' space on Killzone.com. The website tracks all your single and multiplayer stats and presents them very clearly and in great detail. The best part is the battle replay function. Presented in a 2d map you can see the entire match play out and see where you went and killed as it happened. An amazing post op feature that clans will use and devour.