A lot of people miss the point
Seriously, if you review this movie on it's plot, you're a douche without a clue. It's all about the music, which is *beep* awesome. If you judge this movie on the music and still think it sucks, you're a retard. There is no other word for it. The ending scene features some of the most legendary blues artists together on stage. That has never happened before.
It's just a feel-good movie with awesome music, what's wrong with that? You people are taking the movie, and yourselves way too seriously.