Brilliant showcase for all the actors. Beyonce is a little blank but it's appropriate to her character that she come across that way, and by the end she's pulling out all the stops. Eddie Murphy runs the range and has some heartbreaking moments. Danny Glover shines, and Jennifer Hudson is beyond outstanding. Excellent casting across the board.
A central theme of Dreamgirls is the appropriation of soul/r&b music for crossover appeal, but the movie doesn't go into a close examination of the complexities within that situation. What it does instead is to show us characters embodying different paths taken, and how they're each affected by this aspect of the business. When we see Hudson's character painfully pulling her broken life together to shine in her strength at a nightclub, only to have her song turned into someone else's commercially successful glossy disco hit, the contrast speaks for itself.
It adds resonance to be aware of the people these fictional characters are based on, because the movie does interesting things in showing the changing times in which parallel events took place. But don't go into it worrying too much about how accurate the storyline is or where it diverges from reality, because ultimately it's a musical, and musicals are their own worlds. They envelop you and take you into a place of showy ritual and symbolism, where all the things that usually get held in, or said wrong, or stuttered out in drab bits and pieces over a lifetime, can pour out in glorious song, making them less realistic, but resoundingly true. Catch this one on the big screen.