Killzone has a decent campaign and good multiplayer, but terrible AI and a load of technical issues really hurt.
The gameplay is pretty solid for the most part. There's a good variety of weapons, and each character is different in the single player, which makes for a different experience and adds some replay value. It doesn't have the most satisfying shooting, but it's still fairly solid. The aiming feels a little weird at first, but you'll get used to it. Using the sniper rifle is kinda annoying, as when you zoom in with it, the aiming acts like it would in Perfect Dark or Goldeneye 007. You also can't move when you're zoomed in with the sniper rifle, or else it will un-zoom it. The worst thing is the terrible AI. They do a lot of standing around, and like to walk in a straight line, which makes them really easy to kill. Your teammates are pretty stupid too, although they can sometimes be useful for killing enemies, and they can't die either. If they could die, you would have to restart missions a lot. Another frustrating thing is the checkpoint system. Sometimes you don't even get a checkpoint in a section, which will make dying a very aggravating thing. Usually, you'll get a checkpoint about halfway through the mission, but some of the missions are pretty lengthy, and you will have to play through a lengthy and frustrating part again. The campaign is 11 missions long with a couple sections in each. It's fairly lengthy, at over 10 hours. It's very repetitive, because of the AI. I have no problem shooting the same enemies over and over again, but the AI makes it repetitive and kinda forgettable.
The multiplayer is considerably more enjoyable than the single player. There's six different modes to play, and 8 different maps. The maps are good overall. Some of the modes are no fun when you just have AI playing, as they suck at defending and destroying targets. I never got to play online, but I imagine it's a lot better. The gameplay is still decent overall, but it's very flawed.
The graphics are a big mixed bag. The texturing is decent, and the characters look good up close. When you're far away though, they're just a blurry mess. The best thing about the graphics is the great art style. The weapons also look really good, and I dig the animations for reloading and switching weapons. The animation is okay overall, but it can look a little weird at times. The death animations are terrible, as bodies will occasionally get caught on walls and stuck in the ground. They also die in unconvincing ways. The graphics are riddled with glitches, and there's a LOT of pop-in. The frame rate is probably the biggest offender. The game likes to chug and it's very easy to see that the frame rate is slowing down when you just move. When you're standing still, it looks good, but that changes when you start to move.
The sound is considerably better than the graphics. The voice acting is decent, with some cheesy dialogue here and there. The weapons sound good. There are some occasional glitches though, like when one of my guns made a completely different sound than it's supposed to when I fired it. The music is good though, and kinda has a epic feel to it.
There's quite a bit of replay value. The campaign is around 10 hours long, and you can always play through with the different characters. There's also 3 difficulties, which don't make much of a difference. The multiplayer will keep you busy for a while, as it has a decent selection of maps and modes. The AI isn't good here either, but it's slightly better.
Story: Interesting and kinda unique. It's not told well though, but the voice acting is decent. 6/10
Gameplay: Fun, occasionally frustrating, and repetitive due to poor AI. Pretty good variety of weapons and environments. Multiplayer is solid and campaign is good. 7.5/10
Graphics: This is where the game gets really messy. The frame rate is pretty bad, and there are lots of little glitches that really start to add up. At least the textures are decent for the most part, and the reloading animations are nice. 6/10
Sound: Quite a bit more consistent than graphics. Solid voice acting overall, good music, and the weapons have a nice punch to them. 8.5/10
Value: A lengthy, but repetitive (and mostly fun) single player. The multiplayer is quite a bit better with a solid amount of maps and modes. Having different playable characters in the single player is a nice touch. 8/10
Killzone is a good shooter (not great, just good), but it's flawed and occasionally frustrating. Sometimes, it feels a little rushed, and it could've been so much better if they took a couple more months on it. Instead, we get a good shooter riddled with issues. There are much worse shooters out there, and there are much better ones. Despite that, Killzone is a solid shooter that is worth playing.