I'm still in SP myself. I'm taking my time, as you can tell.

The AI isn't super-duper but it is decent. The difficulty from easy to hard doesn't match up.... easy is to easy and hard (vet) is to hard. Not that I don't mind a challange, but it will take me longer on the hardest setting. I think it it kind of silly how your squad can't die, I guess they are needed for the story or something but its funny to me.
And a few times, I've shot an enemy full of holes and seen him lay still thinking he is dead, only to have him get up later when the next rush of bots comes at me. I'd shoot him, he'd die again- next wave, he gets back up again- and again...
Worst AI goof, I was sooooo far into a mission (you don't get health packs in veteran often), my teammate runs ahead, I don't hear any gun fire so I rush in the building after him- big mistake. He is standing in the middle of the room out in the open like nothing is going on, and there are four Germans stare'n at him until I walk in- I'm dead now. That traitor lives and might have stolen the pocket knife my grand-dad gave me once his German friends capped me good.