Why it's getting mixed reviews...
The city is merely a backdrop to the story. The developers have tried to make that backdrop as realistic as possible, to give weight to the story, but I guess they made it look too good. So good that people expect to go off and do other stuff. That's not the point of the Mafia series.
My advice, follow the plot... don't lose it!!
Bottom line: if you enjoyed the first game then I'm sure you'll enjoy this one. At the very least, judge for yourself.