I cannot enjoy CoD4. I never tried the multiplayer, but that's because I just found every part of it to be too... normal? Mediocre? There was nothing that set it apart from any other game out there.
The weapons are boring and sound like pea-shooters with silencers on them, and even though you can get hit by ten bullets and live, if a grenade explodes anywhere near you, you're either almost dead or dead instantly.
The story didn't interest me, and I really tried to enjoy the game. I tried twice, actually, and I just didn't find it fun. It was just... shoot them guys, then those guys, then listen to your squad leader, then shoot some more guys... it was just same old same old. And I have games like L4D and TF2 to play online, and that's plenty for me.
I think my biggest complaint is the guns in the game. They literally put me off playing, because they're so boring and so "normal", except that they sound AWFUL. I want a gun that almost deafens me as I shoot it! I guess that's because I have an awesome speaker setup, though.
To be honest, I've never really enjoyed the CoD games anyway. They've always seemed a little normal to me; just the same shooter I've played over and over, except with guns I don't enjoy. But the games are quite popular, and I'm not so sure that they're as popular on the PC as they are on consoles.
Anyway, I guess if you like shooting things you could enjoy CoD4. I just need a little difference in my shooters, and for me, CoD4 failed to bring difference to the table. I've never been a fan of war-games anyway, really. I'd sooner be the super-hero rather than a normal soldier, even if the normal soldier can take a million bullets.