The movie is pretty interesting, it shows what it could be like if the south won the civil war and slavery still persisted today. The modern-day commercials like the 'Slave Selling Network' and other products that put black people in their minstrel-like places was pretty poignant. The end of the movie showed that some of those commercials that were portrayed were actually REAL products, some used into the 50s.
However, from a 'what if' standpoint, the history is pretty unrealistic. The movie shows the South taking over all of the North, something that would have been impossible, and something the South never intended on doing. They just wanted to secede from the US and form their own country - they did not have the manpower or resources to occupy and conquer all of the North. If you want to read a more probable history of what may have happened if the South had won the war, check out Harry Turtledove's series of books. North and South remain enemies, fight each other in the 1880s, in WWI, WWII, etc. This makes European history pretty different as well, since you didn't have a united America aiding the Allies in the World Wars. The South under exceeding worldwide pressure from its allies ends up ending slavery, but laws are in place to be sure to keep the black man down in such a way that they are not much better off. More realistic I think.
Still, a pretty good movie.