FIrst of all the graphics look amazing. The vivid colors and special effects make this game stunning.
As for gameplay, it is great and fun. The pace of the game can be somewhat slow. In other words, its like playing a game thats faster than Socom but slower than Call of Duty. Just play a few games to get used to it and i will assure you, you will like it.
Controls can be sluggish sometimes. You may find yourself guessing/asking yourself "Didn't I press that button?" once or twice but it does not happen often. Controls are not customizable which is a major let down but there are very good button layouts. If you are a Call of Duty buff, you may enjoy the Alternate 2 button layout which is almost the same as the default CoD button layout.
The game provides both online and offline multiplayer. Both are fun as hell. Online, you will find that the game will take some skill. It is much harder than the Call of Duty games. You wont experience bad lag in Killzone. You will rarely receive a disconnection error. The timing online (in terms of actions) is pretty much on the money. There is no delay when you throw a grenade, your character will throw it immediately. This is very different than CoD where the lag is so bad sometimes, that grenades are thrown with a 1-2 second delay. Maybe once in a while, you may find yourself shooting a guy forever and getting the kill like a minute later. Melee is pretty much done with no delay also. Overall, the online is pretty solid.
So is Killzone 2 worth it. YES! Buy it now, it is a must have!