Here we have, of all things, a superhero's origin story. The character begins as a downtrodden slave with (understandably) no self-confidence; a powerful mentor guides him to develop his own strengths and abilities; there's even the obligatory awful first attempt at choosing a costume. By the end of the movie, he's walking away from a hailstorm of bullets (even shaking them out of his coat), ready to fight evil wherever he may find it.
Jamie Foxx as Django will not get the attention of his co-stars in flashier roles, but he's very good here; he provides the solid moral center around which Tarantino's manic energy revolves, and communicates a lot through subtle facial expressions, since Django is not always in a position to say what he's thinking.
The men filling those flashier roles deserve the attention they're getting, though. As he was in Inglourious Basterds, Christoph Waltz is spectacularly good; he may be the best interpreter of Tarantino's dialogue ever. Leonardo DiCaprio and Samuel L. Jackson are having great fun as the principal villains, and it's particularly nice to see Jackson doing some real acting after spending too long coasting through too many dumb action flicks.
The ending goes on a bit longer than necessary -- the movie's almost three hours long -- and the final shootout is absurdly over the top and bloody, even by Tarantino's standards. And I could have done without a broad comic scene in which a group of Klansmen bicker about not being able to see through the poorly cut eyeholes in their hoods.
There has been a lot of controversy about the movie's frequent use of the n-word, and there is a lot of it. Tarantino argues that it would be dishonest not to use the word in a movie set in 1850s Mississippi. To be sure, his previous fondness for the word in his other movies set in different places and times can make that argument sound conveniently self-serving, but I think it's a valid one. I don't know how you tell this story, set in this place and time, without the word; you can't have plantation owners talking about "African-Americans," certainly, and trying to avoid the word entirely would require such linguistic contortions that it would itself become distracting.
Given the time and setting, I didn't find the use of the word excessive; others will argue that any use is too much, which is a legitimate moral position, and I wouldn't try to dissuade them.
But for those with a less absolute position, the movie is absolutely worth seeing. The dialogue is glorious to listen to; the movie is filled with striking images; and Tarantino somehow never gets the credit he deserves as a director of actors. (Django's candidate for career rejuvenation is Don Johnson, who is quite good in a small role.) Not a perfect movie, but a terrifically entertaining one.