It's quite short and the later part of the game-world loses the atmosphere that made the beginning of the game interesting/engaging. The second criticism might just be my personal perspective so I suppose you ought to take with a grain of salt. That being said, the story is the strongest element of the game.
Also, unlike the original Mafia, the game is extremely linear. I seem to recall some reviews mentioning this problem was partially resolved in DLC but, as I haven't played any of the DLC, I can't comment on that.
It's not a bad game but never really hits the same highs of the original (or any other great game for that matter). As I mentioned, the story does a competent job executing the typical 1950-1970s American mafia story but I found the game world and gameplay so uninteresting that I eventually wound up youtubing the four or five of the final cut-scenes to see the end of the story.
It's not bad but there are some good recent and classic games that would probably be more enjoyable.