Killzone 2 has to be the best FPS i have ever played. There was SO much hype before the game came out that i was questioning weather KZ would live up to its expectations but i have to say it does with flying colours!
The first thing that strikes you is the graphics. Anyone who says that the graphics in Killzone are not that good need to get a HD TV seriously. The backgrounds, the rough environment, the guns, the AI, you can just tell that Guerrilla Games has spent a lot of time and money thinking about every single detail.
The gameplay is breathtaking, i have read a lot of reviews from people saying that the controls are difficult to get used to but its very simple to change them. If you're used to Call of Duty 4 controls just change the control setting to "Alternate 2" and you should be fine. The campaign itself is very difficult, there are some levels where you have Helghast soldiers coming at you from all angles. The length of the Campaign is also quite long which is good. There are also a lot of different types of guns to choose from with everyone having a different look and feel to it.
The multiplayer (WARZONE) is extremely deep, after you complete the campaign this is where you will spend most of your time. You can create your own clan and invite your friends plus full stats are available in game.
The only criticisms that i can think of is the fact that there are some minor frame rate issues which you will experience in the campaign but its nothing major. There were some multiplayer issues which you would expect from a game this large but they released an update which fixed that.
In Conclusion, i have owned almost every FPS on the PS3 (Call of Duty 4, Battlefield BC, Las Vegas 2 etc) and Killzone 2 is leaps and bounds above all of them in every aspect. If you have a PS3 you have to have Killzone 2. The only game that I can think of that could challenge Killzone 2 is Call of Duty: Modern Warfare 2 or Battlefield Bad Company 2 but we will wait and see.