Amazing, Addictive and Exhilarating! You'll be hitched on it for ages, only 1 or 2 minor problems.
I didn't play online straight away, but once I did I loved it. Once you get some decent weapons on you, you can really kick some ass!! I had been playing Tom Clancy's Rainbow Six Vegas just before I went online, and one feature I would COD4 too have, is that you can dress your own character. I suppose that would mean you would have to take out the teams but I think it would be worthwhile. Another thing is on RSV you can make your own public game, which I think is also a very good thing, because sometimes I only want to play with my mates but I also want to earn some XP!
Even though there are things that in my opinion could be better I still think this is one of the best PS3 games there are! If you have a spare 40 pounds lying around, go and get COD4 straight away!