It's good, but it's predecessor is better. (Updated)
I'll start with the the single player campaign. I thought the campaign was good. The story could have been better though. I thought that seeing more of the Helghast affairs was interesting and thought that the story could have been somewhat good, but it was poorly executed. It's a shame because some of the voice acting was pretty good. I didn't like Narville's new voice, not that it was bad, it just didn't fit him. You may recognize his voice from him being Dexter's dad or Raiden from Mortal Kombat Annihilation. I didn't like the one new character they introduced from the ISA, that being Jammer. I felt like she was unnecessary and didn't bring anything to the table. It was just a way to move the story.
Ok, off the story now. The gameplay is great. It is what you'd expect from a shooter and Killzone after playing number two. It is basically the same, without the overly weighted controls, which I personally liked but oh well. The only new thing I remember them adding was a slide maneuver when you run. You can use it to get to cover if ya want. It is a nice addition. There are too many on-rails sections though. Some are ok, but the later ones just feel tacked on and are more frustrating than fun. The campaign is also a little easier that Killzone 2's. This may not be a big turn off for some, but I didn't personally didn't like it. I like a challenge. I was able to beat this game on it's elite difficulty fairly easily, there were some frustrating parts, but nothing that I couldn't get past. I would say that what makes it easier is the fact that Rico or Narville can revive you, but the parts of the game where they are not with you didn't give me that much trouble either. Gamespot mentioned that the cutscenes detract from the gameplay some, and I agree. It felt like Final Fantasy 13 where you walk for a little bit and then cutscene. It may have not been to bad if the story was executed better and if the sound wouldn't have been lower than the rest of the game. But when you are playing the game it is pretty fun.
The multiplayer is pretty much the same thing you get in Killzone 2, but with some differences. They have reworked the class systems. When you level up you can spend you points unlocking skills and guns. While the classes are the same, some things have been changed about Gaining experience is a lot easier now because kills and other things you do online are worth more points. There are three modes now. Guerrilla Warfare, Warzone, and Operations. Guerrilla Warfare is you standard team deathmatch mode. Warzone is a game brought over from Killzone 2 where you play 7 different game types in one game. Operations is the only really new game mode. In operations, depending on what side you are, you are attacking or defending points of interest. The points are based off of other game types from Warzone, like attacking/defending and point, capture and hold, and others. Operations focuses mainly on working as a team more than the other game modes.
There are other changes to the multiplayer as well. There is no longer the squad system that was introduced in Killzone 2. In the old squad system you partied up within your own squad. Each squad had a leader and you talked directly to you squad mates. You could also spawn on the leader of your squad.Tacticians no longer have spawn grenades, they now take control of Tactical Spawn Areas (TSA). So you now limited to where you spawn. That wouldn't be to much of a problem but spawn camping is bigger problem now, especially if there aren't a lot of TSA's in a map and the other team has them secured. But all in all, the Mulitplayer is pretty much the same as in the previous game.
The Move works well with Killzone 3, especially if you have the sharpshooter. Now there is a slight learning curve with it, but once you used to is you'll never go back, unless you have to charge it and use the dualshock while it's charging. My only complaints with using the Sharpshooter is with the on-rails section and melee-ing. The on rails parts don't feel natural with the sharpshooter, it prolly feels better with the Move by itself, and the melee-ing works, but if you try to do more than one melee thrust it gets kinda hectic and hard to perform. As of this review there is no button for melee-ing, but they are patching one in.
The co-op takes you through the campaign, only with a friend. Your friend does not take on the role of Rico, he takes on the role of Natko from the second game. All of the cutscenes though do not have your friend. There are times the game feels kinda hectic with two people, like when you are in exos, but other than that it works fine. There are some problems though. The dialogue will sometimes cut out, and the sync gets off for cutscenes. And one of the more annoying things getting trophies. To get trophies like 10 brutal melee or killing 6 Helghast, killing 6 Helghast in a row with the sniper rifle without reloading, etc, can only be done with first player. If the second player does a trophy requirement, it won't count. But other than that, and it not being available online, it works fine.
In conclusion Killzone 3 is a great game. If you are new to the series and play this one first, and can look past the awful attempts at a story, you'll love the game. If you have played Killzone 2 you might be slightly disappointed in this game. Either way it is worth playing.