IGN has a review up.
IGN: Mercenaries 2: World in Flames Review
Basically. Everything about the gameplay (except for blowing shit up) is annoying to bad. Bad sound. Bad AI. Questionable design decisions. No mission flow. Bugs and errors.
BUT.
They say blowing shit up is very very nice. Ultimately the redeeming characteristic of the game. Blowing stuff up is so nice, it manages to overcome the bad stuff and make the game good.
Oh, online co-op.
I think its a buy, but I'm going to wait for it in the bargain bin or sale.