I've heard mixed reviews on this game. Can anyone recommend it with a GOG seal of approval?
Well, it is a mixed bag. ;)
It's definitely better than the first one, I hated that. I don't know about this version but in the old version the 40s/ 50s Radio songs were really perfect for the mood. For 80% off it offers plenty, if you into this kind of games, IMHO. I liked L.A. Noire better though (because I am basically much more into Film Noir than into Scarface).
For the story DLCs, Jimmy's Vendetta and Joe's adventure is a series of timed kill missions.
... nothing essential, though I only played Joe's, which is generally said to be better.
The Cars and costumes, well, if you like the designs, they don't hurt.