Like any other CoD game, it's fun in short bursts. I can only play two or three rounds at a time before I get sick of it though.
I bought a heavily discounted Russian copy, and I feel like I already got my money's worth, so I don't really regret it. I might fire it up again this weekend just to pwn some noobs.
I just recently started playing Battlefield 3 though, and it's seriously spoiled other online shooters for me. MW3 was already a "meh" looking game, but after playing BF3, it looks like complete ass. You also gotta love how MW3 has absolutely zero personality or character to it. It's one of the most empty, soulless games I've ever played. Despite what people think of EA, at least Battlefield 3 has some semblance of creativity to it.
I also hate the extremely generic, contrived "WAR MUSIC" that plays in the menu, and I wish there was a way to turn it off, but there isn't. The options in MW3 don't let you adjust music/soundeffects individually.
But, eh... I'm just nitpicking now. Like I said, it's fun in short bursts.