Very underwhelming. Interaction with L4D1 crew is just that really, nothing exciting at all.
Three level campaign, about the best bit was the charge for the safe room at the end of the second level.
Also, Valve have managed to make the AI worse, twice I was left to die by bots stuck inside walls, floors, or their own thick skulls. I know it's a heavily MP game but if you're not going to bother to code bots properly, why have them at all?
I'd give it 5/10. It's not terrible, but i'd be stretched to call it good. Very meh, very disappointing.