Having finished this on Veteran, campaign opinions only:
Overall: B. First let me say that those critics claiming the campaign was a "miserable experience" or "it is just not good," etc., are full of shit as far as I'm concerned. This was definitely a solid campaign. It's not what it could have been and lacks the impact of its predecessor, but it definitely delivers and is worth recommending to any shooter fan.
Overall, though, I was personally disappointed especially with the first half, though the game really turned around starting with the ice level. The sense of gritty realism from KZ2 has dissipated, as has the atmosphere, as has the level design and the feel. The campaign feels like they were simultaneously more ambitious (more locations, types of missions, etc.) yet had far less time to polish it and make it really strong. Too much pandering and trying to please everyone. I don't blame them for trying something different: they likely thought they'd get backlash if they just did KZ2 part II; but I just don't think they're experienced with this CoD-style globe-trotting thing to make it work with just 2 years of development... it's not their style. CoD has the "shooting gallery with pretty set pieces" thing covered -- Guerrilla should focus on making a nice tough tactical shooter. KZ3 was a step back in terms of the latter, even if did pick up some CoDisms.
Gameplay: B+. There's good news and bad news. Good news first -- all the glaring issues with KZ2 have been addressed. At no point did this game have any momentum-killing ultra-frustrating sections like KZ2. No endless waves of enemies, no huge difficulty spikes. Therefore at no point was KZ3 as bad as KZ2 was in a few isolated spots (for example, the AA gun and the pre-Radec fight). The other major problem with KZ2's campaign were the friendlies, who didn't kill anyone and constantly died, having to be revived. Not only are they competent fighters now, but they will even resuscitate you if you die.
So, that's good. The bad news is that this game is just nowhere near as intense as its predecessor. It is far, far easier overall (I seem to remember having a slightly harder time on Trooper in KZ2 than Veteran in KZ3). That's not necessarily bad, but combined with the fact that approximately the first 1/3 to 1/2 of the game features no memorable firefights (in fact, I can barely remember anything before the stealth jungle section), it makes for a far less exciting experience. Honestly, all of the escape from Pyrrhus is rather dull, when it should be pulse-poundingly terrifying. There was this standard fight against the snipers (sniper on the left! sniper on the right corner!) which was so by-the-numbers I was honestly rolling my eyes. This is a big contrast with Killzone 2, which pretty much exploded from the get-to without such gimmicks.
Excluding the stealth section (which was fine), the game's low point was the jungle area. It was rather ugly and ended in a confusing standoff. I was about to give up on the game, but then it really improved. The demo level (which the demo did not fully represent) was great. It was fairly open and vertical. Everything from that point on was at least solid. Finally the game's enemy A.I. got a workout. The MAWLR set piece was awesome (though admittedly not nearly as terrifying nor difficult as one would assume just looking at the thing). Overall, this game was definitely a step back in terms of the bread and butter level design which was so good in KZ2 but just par for the course here.
Unfortunately, even just the basic game mechanics were one step forward, one step back. Certainly the shooting is more responsive and easier. On the other hand they've added really blatant auto-aim that's almost CoD-like at times. Also all the main automatic weapons seem way too similar to each other now. The two ARs are seemingly identical, and the SMG is not much different. Even the LMGs seem way too usable at long range. Ammo was an issue in KZ2, but ammo crates are EVERYWHERE now, which sadly removes a lot of the tension. Ammo crates and weapons racks also appear in nonsensical areas.
Finally, there are the vehicles and on-rails sections: namely too many of the latter and too few of the former. It's not that there were that many but that it seemed like the game relied on them to provide the WOW moments... and with the exception of the MAWLR fight, it really didn't. Honestly, they should mostly abolish on-rails segments. Put in some real vehicles instead, and make those actually challenging like the real firefights instead of making them pure "reward" sections where you get to slaughter everyone.
BTW I recommend changing the control layout to Alternate and set Crouch/Cover to Hold (instead of Lock). Much nicer if you like to use cover IMO. Easier to get away if you get grenade-spammed.
Story/atmosphere/voice acting: B-. Again a lot of hyperbole on the criticism in the press on this (though admittedly I was playing in Russian; in English a lot of the battlefield chatter seemed poorly done by comparison). This is not a bad story. In fact it's fine. It was clear what was going on, and it set up some interesting situations and a decent dramatic arc, for a shooter. The Helghast in-fighting was effectively conveyed, and I actually felt a certain kinship to Narville and (gasp) Rico this time around.
The problem was the constant interruptions. It's fine in a slower-paced game like an MGS. In Killzone 3, though, there were way too many times when I'd get through a 5-10-minute-long section of brutal fighting, only to suddenly walk into a cutscene when my gun is involuntarily lowered and a bunch of exposition is shown. Games can do cutscenes effectively, but it has to be either infrequent, or it has to be blended into the game (see Uncharted 2). It was way way WAY overdone here, with tons of sudden transitions.
So yeah, on a pure plot and performance level it was just fine. But in terms of how it affected the gameplay... it was pretty bad. Take Resident Evil 4. That game had a terrible story (much worse than this), but it didn't get in the way of the gameplay. It did here.
Oh, and the ending was AWFUL! What the hell!
Graphics: A-. On a technical level, superb and far ahead of any other shooter on consoles (that's released, anyway). Clean image quality, excellent animations, mind-bogglingly complex environments (though mostly inaccessible for gameplay purposes), at times shocking amounts of simultaneous activity, all without losing smoothness (except for this one bizarre pathological 5-fps area at the end of an early level, which has to be some bug). I mean, I didn't have an FPS counter running, but with the exception of that one fight, I didn't encounter any noticeable frame rate drops ever.
On an artistic level, it definitely loses its identity. For all the me-too criticism of KZ2's "brown" color palette (which wasn't even true... the game had quite a varied color scheme over its overall course), it had its own identity. It was different from everyone else. All those post-processing effects combined with the oppressive lightning and dark skies of Pyrrhus in KZ2 are kind of gone. Instead the game goes for a brighter look. It worked. It's not as unique, but it's usually quite impressive -- once you get past Pyrrhus and the jungle. The jungle itself, however, was terribly ugly and didn't use the engine's strengths at all. There was actually a time in this game where I thought I might prefer KZ2's graphics to KZ3's. Again, all that disappeared starting with the ice level but still.
By the way, the loading hitches from KZ2 are mostly gone. The are some, sometimes, but they're barely noticeable to me.
Sound design/music: C. What a mess! I don't place *that* much importance on sound in games, but honestly, WTF? At default settings, I had to turn the volume up to hear the cutscenes, then when the gameplay started again, people's voices and sound FX *blared* at me. Eventually I had to set all those thing to like 60% volume to even this out a bit. Then at times during the gameplay, people's speech would be whisper quiet then suddenly get nice and loud. No, it's not some technical problem with my setup. The sound design was just not properly done in this game.
Value: B. I finished on Veteran in 6.5 hours not counting deaths. So that's probably something like 8 hours total. Shorter than KZ2, much shorter than Reach (for me), longer than the CoDs. So not bad. The lack of online coop is *sigh*, though to be fair, I am not really inclined to replay through this.
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I also played 2-3 hours' worth of multiplayer. MP is not really my thing, but I have to say that this, to me, was really enjoyable, and I will be playing again. I think those that were unhappy with KZ2's online should find much to like here. All classes are unlocked, a constant stream of rewards, simple matchmaking, simple party system, tight controls, less health (not like CoD but closer), no more 32-player clusterfucks; Warzone is back and still rules. Haven't had a chance to try Operations yet. The menu system is really solid and responsive though also way simpler than before. This, to me, was just so much more accessible than KZ2 was. No more trying to make it as a grunt before using some real classes; I feel like I can contribute to my team (and points to myself) as a Tactician from the get-go instead of basically getting reamed by the game in the beginning, even if I'm not a great shot (0.7 KDR, baby!).
Already I felt a couple of problems, but they are not game killers. First of all, I got stuck on Salamun Market -- the game refused to put me into any other Warzone maps. I had to pick a different map on purpose to make the game stop doing that. I also couldn't find a detailed stats screen -- how many matches I've won, what my favorite weapon is, etc. -- not in-game and not on Killzone.com. I assume people will have complaints about balance, and I haven't played enough to say anything, so all I can say as a noob is that it was fun. I'd love to play with some people on here in a more organized way (it'd be nice to have an engineer who can repair ammo crates near by Spawn Areas).