Rudy Ray Moore is an interesting guy. He's one of those people who's image has continued to grow after his death: his spectacularly raunchy comedy albums were too filthy to advertise in the 1970s, but they sold by word-of-mouth just fine - and now, because he rhymed over music, he's "the Godfather of Rap" (and they kind of have a point). I think a big part of his success was simply being transgressive: these days his records only sound a little rude, and not all that funny. But the people who made this movie had multiple records to mine for material to make the script, and the end result is very funny.
Eddie Murphy plays Moore in the movie: he wants to be famous, but his life isn't going anywhere. Until he repurposes some backstreet rhymes and jokes and puts on the persona of Dolemite. And that serves him fairly well, but he thinks he should be on the big screen. So he mortgages his future to make a badly produced movie.
I knew the story, I knew where this was headed. (I'll give you a hint: the "Dolemite" movies are crap, but they're a kinda funny blaxploitation that was fairly successful.) But the success of this meta-movie is in showing the good and the bad, the failure and the success. Yes, he was making a crap movie - he didn't have much money, and he didn't know what he was doing. But he was doing it because he wanted to entertain people, to make them laugh. And in the end, he succeeded.
It's good to see Eddie Murphy back on form: he's successful in this role because the humour comes from the character he plays (with a good script), and he's just ... being Rudy Ray Moore. And holy shit, Wesley Snipes may finally have found the sense of humour he lost 20-odd years ago. This was just a fun movie to watch.