I got it.
It's somewhat like GTA, in that it's an open world game. But it's a lot more realistic than GTA.
It's pretty good. I don't regret getting it. But then again, I love the 50s. The look, the music, the cars. Trench coats/overcoats are so win.
The cars are the biggest difference from this game and GTA IV. First off, not every car is exactly the same. Well, 98% the same. Every car in GTA gets to the same exact top speed, some just take a little longer to get there. Some handle slightly better than others. But most cars are exactly alike. This is completely untrue in Mafia II. Each car responds differently. Most cars have the same top speed of near 100, and take forever to get there, but there a few cars that are noticeably faster and a lot more responsive than others.
I would rate it 4/5. There needs to be more side missions. Otherwise, this game is amazing.
Is it worth it? $50 is a steep price. Perhaps when it is closer to $30. Unless you absolutely have to have it, it's not really worth it right now.