To get started I'll add my own vote, can only be "outstanding!" for me.
This game is just so much fun that it's been giving me the regular late nights that only the original GoW before it inflicted.
I have to admit I haven't really touched the single player, that's not what this game was designed for, but the online co-op is just pure gaming bliss and to me is just a classic example of the main reason why I love gaming so much, it's fun with a capital F!
Probably my only reservation after playing the demo was how the game would hold up to repeated playthroughs, would I get tired of it? The simple answer after pilling countless hours into it is a definate "no chance!" Just so adictive and if you can get three other regular players then it's pure gaming bliss. I'm still perfectly happy playing through the game on normal difficulty but those tougher difficulty levels are going to add a huge challenge and will take alot of persistence to get through, and this is one game I will be determined to do just that(and I don't usually like being challenged too much).
The director really is a stroke of genious IMO, it really is the secret to one of the games main strengths, replay value. If the enemies were placed in the same areas each time then for sure it would soon become repetative, but the fact you never quite know what to expect keeps the game entirely fresh and I think for the average person, if you get hooked with your first play, you will remain so for a very long time.
Aside from the co-op(the main mode IMO) and the single player(where bots take over your team mates) the multiplayer verus is also something pretty special. 8 player humans versus infected is something so different from just about any online game I've played before that I love it. Taking the roll of the infected bosses is fantastic fun and it really does round off the package a treat, something I really expected to be a discraction of an afterthought really surprised me with how well it plays and how much fun it can be.
Oh, and one last thing, the "take a break" feature in the online co-op, wouldn't be without it now, phone or doorbell rings, no problem, select "take a break" and the AI will take over your character until you return and pull the right trigger, at which point you jump straight back into the game, damn useful(and Idle:Tetlee is pretty good too

).
So there we have it, the weakness of single player is a complete non-issue for me in these days where I prefer to play online. It was never meant as a single player and just as some classic games are unaffected by not having an online mode, the same principal should be given here(in fact less so as it can be played single player, and can still be fun played that way too). Take the game as it is, a straight up multiplayer blast, and it's just perfect IMO, sits right beside GoW(1) as the best multplayer experience I've had!