I just finished writing a rather long-ish review about Mafia II on the site. You are welcome to read it, but know that it's from the point of view of someone who really, really enjoyed it. I had zero expectations, no hype at all (I'm waiting for completely different games in this period), so my vote was not influenced from other factors. I bought it out of curiosity (and of course with some positive reviews backing my decision to buy it up. Freshly released games cost too much to be bought blindly).
I think it's worth buying unless you're not into following storylines. Of course, do not expect any kind of sandbox mode. It's a railed adventure. So I also understand that it is not for everyone.
Myself, I prefer theme-park experiences over free roam and sandbox, because it adds to the quality of the narration. Another person may say that Mafia II is linear, dull, and gives no freedom to the player, thus considers it junk. It's a matter of perspective.