I just bought the game yesterday, and so far it's a fairly decent game. Most of the campaigns objectives consist of "Defend this spot, eliminate hostiles, make manly Rambo reference" type missions, but otherwise it's good. They do tend to send you off on your own a lot, giving it a sort of "Superman" complex. I was slightly disappointed with the lack of secondary fire for weapons like in the first game.
The multiplayer is, of course, the area the seem to have put in most of their effort. It has a leveling system like CoD4, except you only get experience from kills, not finishing objectives. Each map (Which there are surprisingly few of) plays through all of the game types; Assassination, where one team must protect one of it's members. Attack and Defend, where one team must plant some bombs and defend them until they go off. Assault (I think) where one team must defend a spot (Which you spawn really fucking far away from) from the enemy. Theres a few more, but I can't remember them, and they're really nothing new.
Back to the leveling system. The higher level you are, the bigger a variety of weapons and equipment you can use. You also unlocked classes; Medic, Spy, Engineer, Sniper.... This sounding familier to anyone else? Although most of the classes have the same skills as those in TF2, it really lacks the strategy element. The Engineer can place down a few defensive turrents, but one person can easily unload a clip into it and destroy it before it notices it is dieing. Theres also the "Technician" which can throw down green smoke, AKA, can set a spawn point in strategic locations, although, the word "Strategic" seems to be lost on a lot of the people who choose this class. More often than not, you will look at the map, and the spawn grenade will be nowhere near the objective. Luckily, if you're in a squad, you can spawn near your squad leader, who, unless he likes standing in a corner doing nothing, will probably be doing something useful.
All in all, I give the game a B-
Not the revolutionary doorbasher it promised to be, but still very worth buying. I've only played the game for one day however, so updates are still very possible. Either way, Killzone is still and will always be the Halo killer.