It feels like a good game. It just plays like crap on my computer, and that has tainted the experience. I think it's a nice CO-OP to play with your friends, but once you get it done once or twice, it's not really as good. You have to play it on the highest difficulty, because otherwise you'll complete it, and then you'll be satisfied - if you're keep on trying to beat it, the fun lasts longer. I don't think the versus mode is going to add much in terms of longevity.
I'm a TF2er, as many of you know. Is L4D going to pull me away? I'll probably get the game, so I can play with friends, but from what I'm looking at now, it's going to be a 1 month hit, and then it will disappear if Valve or the community do not make some *good* maps and additional content. This is the sort of game that will only go as far as Valve adds content, because I don't see hospital or something turning out like de_dust2 or 2fort, a map that's just played over and over. But I was cynical to begin with, so I may have bias. I still think I'm going to remain a TF2er until a game like Battlefield 3, or maybe Quake Live, Battlefield Heroes, or Starcraft II, comes along. I haven't played it *that* much, so my opinion still may change.
This game is broken without friends, period. Actually, to put it in more general terms, it's broken if you do not have a microphone, and everyone doesn't have a microphone. It just isn't as fun if you don't. It's the sort of game that requires teamwork to succeed, and not only that, since its smaller than TF2, it's easier to have teamwork.
So what should you take away from this review? Well, it's definitely going to be a good game. So go download the demo, now. But I can't help but maintain some doubt that this game will be the multiplayer success its predecessors were.