Killzone 3 is a fantastic shooter; anyone who says otherwise has either not played it or is simply trolling.
As always, I'll start with the graphics and sound. The graphics are simply the best there are on consoles. The new Crysis 2 does rival it in some areas, but for the most part the lighting, particle effects, and level of detail here are far beyond anything offered in other games. The character models are particularly impressive, due in no small part to their huge size and subsequent level of minute detail. The lighting in the game is top-notch and extremely nice to look at, and the environments, though a little generic in design, are also rendered nicely. I don't want to spend too much time on the graphics since I feel that Killzone has gotten a reputation for being visually stunning but mediocre in other ways and I don't wish to perpetuate that stereotype. Suffice it to say that Killzone 3 is quite likely one of the best looking games you've ever played.
The sound is also tremendous - I found myself having to turn down my 5.1 headset to avoid blowing out my ear drums during the gunfights. The weapons are punishingly loud, explosions are earth shattering, and the ambient noise does a lot to really bring you into the fight. In my opinion, Killzone 3 is only topped by Bad Company 2 in terms of audio experience and sound effects.
The music is also good--I really can't say enough about the main menu theme song. It is incredible and may actually rank in the top 5 only-for-games pieces I've ever heard. My lack of excitement for the rest of the soundtrack is less due to it's lack of quality than it is to my disappointment with it not being as amazing as your first taste of the game's music. Still, the rest of the soundtrack is solid and, at times, quite nice to listen to.
The storyline is the same as nearly any other shooter in most regards, but it is strengthened by the fact that the backstory for the Killzone universe is second only to games like Bioshock and Mass Effect. Seriously, at some point go online and read the whole synopsis of events leading up to the games. You will be floored at how much thought went into the overarching narrative here. In fact, you may wind up liking the Helghast (potentially the coolest shooter enemies around anyway) more than the ISA. Anyway, the story itself is pretty standard stuff - hopeless mission, final redemption, yada yada yada. It serves the purpose of moving the game along, but that's about it. The ISA soldiers from Killzone 2 are back and less interesting than ever, although now they are supplemented by the uber-irritating Capt. Narville, a move that I suspect was meant to draw the gamer's ire away from the the still-awful Rico. Rico doesn't die, by the way. Abandon all hope.
I actually wish that Geurilla had spent more time on the interesting political dynamics between the ISA and the Helghast and why they hate each other so much; the civil war story with the Helghast leaders just falls flat due to weak characters and a feeling of being forced. Both sides of the conflict are caricatured with stereotypical leaders like the brutish military commander and the soft-but-smart businessman. Neither side is particularly compelling, and although things start getting more interesting as you near the confusing and downright abrupt ending (which leaves serious questions about how the series will continue), they never really grab you in a way that will make you care what happens.
Fortunately, however, the gameplay is fantastic and although Killzone 3 is, at its core, a corridor shooter, it is a very good corridor shooter. Battles are tense and you are really forced to earn every inch you take. This is not an easy game, and you will be severely punished for any attempts to run and gun or act like G.I. Joe. I only really hae two major gripes about the single player experience. The first is that although the environments are more diverse than in Killzone 2 (the arctic and jungle levels in particular are quite impressive), there is still far too much burned out urban fighting for my liking. I know Helghan is supposed to be a harsh, unforgiving world, but I don't think that always has to translate into boring junkyards or shattered buildings. Second, I could have done without the on rails vehicle sections. A few are relatively entertaining but there are too many of them in the game and even in the ones that give you control are so linear that it's hard to get excited about it. The worst offender is the final battle of the game, which was absolutely awful in my opinion. These aren't game breakers, but they certainly didn't help an already slightly stilted experience.
Killzone has never really been a single player experience, though, and so the real draw is the multiplayer. I'm happy to say that despite some hiccups, the game delivers in spades on this front. The core gameplay is roughly the same as the campaign, with satisfying gun fights and the addition of well balanced class-based abilities like disguises, invisibility, and the use of turrets and robots. All the classes are well designed and each has a useful role in any battle. You will most likely find yourself needing to switch frequently to meet the needs of a given battle, an occurence indicative of a well-designed and balance class system. Certain powers (disguise in particular), may seem a bit irritating at first, but as you progress you will unlock more powerful abilities to counter them. Everything has a counter, and that is a large part of why Killzone 3 is so compelling.
It is also in the multiplayer that we get our true taste of the newly redesigned Killzone controls. Gone are the ridiculous lag times between input and action of Killzone 2. Gone is the super heavy, I'm-wearing-an-anvil-as-a-helmete feeling. The controls are now faster, more fluid and, most importantly, better functioning. The game still retains a heavier, unique feeling, but it is far more manageable for those not well versed in Killzone to pick up and enjoy. Move controls are also implemented, but the fact that I loathe motion controls has prevented me from buying the Move and thus from trying it out here.
The main game mode is still Warzone, though I for one am not a big fan of the chaos that it tends to produce. In addition, some Warzone maps (Corinth highway being the worst offender) experience MASSIVE framerate drops when fully loaded with 24 players. These drops are huge - we're talking into the high teens and twenties here - and basically make the game unplayable on those maps once the room fills up. Other maps, such as Turbine Concourse SE-6 or Bilgarsk Boulevard are extremely well designed and flow nicely. I am still a little baffled as to why we can't play some version of any map on any mode; the current only certain maps for certain modes model seems a little restrictive. Still, for the most part the maps succeed in their given modes.
Where Geurilla really messed up, however, was with the Operations maps. Operations is a new mode that plays much like BF's Rush, but with the addition of cutscenes showing off the top 3 players. It is phenomenal, a stroke of genius. However, the inclusion of only 3 playable maps was a huge oversight on the devs' part. Maybe they didn't think it would succeed, but it is the best part of the game and the only part that many, including my friends and I, actually play regularly. With the recent announcement that new maps for the mode are "unlikely" for whatever reason, it is safe to say that this mode will be going stale very quickly. That's really a shame, because Operations is far more rewarding and fun than Warzone and an increased focus on it could have taken Killzone form being a niche exclusive series to being a massive hit. I do not understand the decision to not support the mode, and in some ways I see it as a slap in the face from Geurilla, but I certainly hope they either change their minds due to overwhelming demand or at least include a fully developed version in Killzone 4.
Overall, despite its failings, Killzone 3 is an outstanding shooter that deserves your attention and cash. Go buy it. You can thank me later.