After playing through about half of the campaign and spending quite a few hours on multiplayer, I can say that I have pretty mixed feelings about it.
For everyone that's had problems online with hit detection, I can completely agree that it's been pretty bad. I've had too many encounters where I've gotten the jump on the guy I'm trying to kill, make no mistakes, and still end up getting killed. Watching the kill cam, none of my bullets are hitting the dude. There's not any lag, and I'm aiming straight at the guy. I mean seriously, what the hell. I've also been shot from around corners like YTP said, and it was really frustrating. I also just can't seem to find a gun that I can be consistently good with, but I'm sure that if I play for a bit longer that I'll find something.
On the bright side, everything about the campaign has been brilliant so far, and I've extremely enjoyed it. Even better than the first Black Ops campaign.
I'm not a big fan of Zombies, but everyone I know says it's a lot better now than it used to be, so I might give it a shot.
As far as how good online play has been, I'd have to put the COD games I've played in this order: MW2 = BO > BO2 > MW > MW3 = WAW
So I mean, it's not ridiculously terrible, but I still don't really have a reason to play anything other than the campaign.
EDIT: I might be exaggerating a bit in that last comparison; I just really hate MW3.