Play for the excellent story, just marred by the lack of other things to do.
Mafia II is good enough to be considered the Godfather of Games. However, like the movie, the story plays out very linearly. Fail a mission, and you have to restart again. There are 1 or 2 side quests in between chapters but this are very few. However, the story is top notch, and the production values are very good. After a while you will start to care about Vito, and his best friend Joey. I just love the Italian accent, similar to what you can get from the other Italian speaking game, Assassins Creed II.
As a sandbox game however, Mafia II disappoints. Empire Bay is just waiting to be explored but you can't do much other than patronize a few shops, randomly shoot some passerbys or just plain carjacking. Unlike the GTA series, there is hardly any incentive to explore the City besides marvelling at the scenery. More missions that capture the whole of Empire Bay would have raise this score to 9 and above, but unfortnuately there are none. What Mafia II needs are more gang wars, taxi missions, police chases, bar shootings, roughing up the neighborhood and so on. However, you're limited to robbing 5 stores in 5 mins, finding Playboy posters or stealing 5 cars just for a trophy or two.
I would still recommend Mafia II though as it provides a solid 10-15 hours of good fun with an excellent story to boot. The add-on Betrayal of Jimmy does extend the gameplay somewhat, but this should be in from the beginning and not for an addiitonal 10 bucks.