This movie is certainly worth watching if you're an Altman fan, or a fan of revisionist Westerns. The performances are great (as per usual when Altman is at the helm) and the movie is entertaining enough on its own merits.
The two biggest flaws, though, are these: Compared to most of Altman's films, much of the dialogue in this movie is very "stagy" and theatrical. I suppose it's supposed to be that way because of the questions of "myth" and "legend" that the story concerns itself with, but my impression was that such theatrical-sounding dialogue didn't mesh well with Altman's typically naturalistic style of filming.
The other problem I had is that the whole subject matter -- myth vs. reality, history vs. reality, show business vs. reality, etc. -- isn't really explored with any depth or subtlety. We're constantly being reminded that Buffalo Bill is a man who created his own legend out of lies, and that that is the basis of modern show business to this day, but really, that just didn't strike me as being a particularly insightful observation. This is hardly the first movie to point out that lies are often more "real" (or more attractive) than the truth, and Altman doesn't seem to bring anything new to the table.
Still, it's Altman, which means it's well-made, entertaining and beautiful to look at. I don't think this will ever be considered one of his major works but it's certainly worth a look.