Killzone 2 is the best in series, with graphics only possible on the PS3. However, the story and gameplay is quite dull.
Firstly, you can't talk about KZ2 without mentioning the fantastic graphics made possible by the power of the PlayStation 3. Everything from lighting, to dust and smoke looks absolutely top-notch and really gives the game a great sense of realism. Character models (including facial animations) are brilliant and the weaponary looks and fires great too. All of the environments are also very well built, however this doesn't disguise the fact that the levels are very linear for a next gen game.
The next big plus point is the physics and motion in the game. The Helghast enemies die very realistically - often clutching at leg / arm wounds. The spin round when you shoot at them from close range due to the force, but simply slump forward or fall back when shot at long range due to the decreased force (as would happen in real life). Vehicles also move very realistically, rather than gliding over the ground they seem to really stick to the surface - and the airborne vehicles fly well too. All this adds to the hyper-realistic feel of the FPS.
Another good thing I found when playing the game is the limited HUD. Often in next-gen games it is thought of as being extremely fun and modern to clutter the screen with mini-maps, maxi-maps, weapon status, health meters, mission objectives etc. etc. However, in Killzone 2 there is only ever ammo and grenade level in the botton right corner and a mini-map in the bottom left corner (with your health indicated using the good next-gen feature of "screen blur" and blood blocking your view - found in COD games).
When reading this you have probably realised that I am a huge fan of realism in next-gen games - which is why you may be surprised to find that I thoroughly enjoy playing the mad and often silly world of online KZ2 (i.e. people killing other players from a metre away with a rocket launcher). Despite slightly unrealistic elements, the online mode offers a variety of different modes with just the right amount of maps - although more would be better and I'm sure this issue will be dealt with following the release of some DLC on the PSN.
Bad points coming up in a quick paragraph (as the game is generally excellent). Firstly, next-gen games should be much more open than KZ2 is during the campaign - linear, run and gun missions being the order of the day (although a variety of mission objectives does keep the campaign flowing despite this tedious gameplay). Secondly, the script for the game does begin to grind due to the inclusion of not remotely funny humour (i.e. copious amounts of jokes related to intimate relations with each others mothers shared between the characters). I understand the macho men from the ISA (which unsurprisingly is comprised of nothing but American soliders) are tough alpha males but does this mean the script needs to be so predictable and at times silly? No, it does not. Lastly, having played many hours of COD over the past few years, I was surprised when I started to play KZ2 and my character (Sev) seemed to be running so slowly it felt as though his pants had dropped to his ankles and he could not in fact move his legs (which could quite possible be made of lead). It made moving from A to B quite tedious - even the "sprint mode" isn't that useful as it takes so long for Sev to point his gun back up at the enemy after sprinting that your are likely to die very quickly in the higher difficulty levels if you sprint a lot.
Overall, despite the aforementioned niggles (which probably only get on my nerves as I am very particular about realism and FPSs in general) Killzone 2 is an amazing game which should be part of every PS3 owners collection. It really showcases what the PS3 can do - believe me when I say the graphics and physics are brilliant - and is also just very fun to play. If you've never been particularly fond of FPSs, Killzone 2 will convert you and you'll love it! If you are fond of FPSs, I don't think I need to tell you that you'll love it!