Same old game...
I'll be honest, I have only been on multiplayer. This is the reason I purchased the game. The single player campaigns up to this point have always failed to deliver in my opinion but I cannot comment on MW3's campaign
If you've ever owned MW, MW2 or BO then you'll be able to jump straight into the multiplayer here with no issues... and that is just the problem. The game has become generic. The maps all look like something you've played before, the graphics are not all that better... everything about the MP game seems like its been done before. OK, they've introduced 1 or 2 new features (prestige points etc) but there is absolutely nothing new or ground-breaking here. I'm glad that I didn't have to pay full price for the game as it's more like an expansion pack than a fully fledged new game.
EDIT - after playing the MP game for a lot longer, my opinion that the series has become boring, generic and predictable has grown. This is truly a mediocre game and does not deserve the sales, the hype or GOTY