After I got COD4 just after x-mas I found the game with amazing online, I didn't start the campaign for about 1-2 months after I got it and it was one of the best single player I have played in along time. Yes its short on normal mode you can do it in about 4-5 hours but on veteran it's a different story. It's like playing hardcore on the campaign.
It was the first thing I did when I played the game and it was great!! Though after 5 months of playing I got a little bored of playing but that all changed with the new map releases 2 days ago. It had me realise what an amazing game it is! Although when you hit level 55 there's not a lot to do part from get gold guns which can be quite tricky at times but I have to say if you want the full experience of COD4 you need to join a clan, I join one call Team Ic3 within the first week of play and they are a good bunch of guys. The best bit about a clan is playing "scrums" is great fun, COD4 with tactics.
It's the best FPS shooter out at the moment. If you liked COD2 then you will love this game it's a must have for all gamers.