I watched it because I'm a sucker for music documentaries. Definitely learned a few things (am I the only one who didn't know she was a backup singer for Michael Jackson?).
I have two observations:
1. The filmmaker is keenly aware of the criticisms against her for being purely self-interested and (as a consequence) for passing off collaborative work as her own. It definitely reminded me of the Robbie Robertson documentary... who faced the same charge. Both documentaries spend almost zero time explaining the basis for the criticism, but spend extensive amounts of time defending against it. It makes for a kind of weird protesteth-too-much movie.
2. The film opens with Sheryl and Brandi Carlile stating very emphatically that the music business is much harder to break into for a woman, and that women have to be more talented and harder-working than their male counterparts. Which is a fucking silly and demonstrably wrong assertion. It was completely impossible to break into the traditional industry (record labels) in the 90's if you were overweight, ugly, or old... regardless of gender. If you were a hot young woman, on the other hand, then all kinds of industry doors were (and remain) wide fucking open -- doors that are hermetically sealed to the unattractive (no matter the gender, no matter how talented).
I think the only Sheryl Crow record I ever bought was TNMC. She's definitely got talent as a singer, writer, and performer, but she doesn't blow me away on any of those individual skills. My wife cannot stand her vocal delivery on a lot of songs (like "All I Wanna Do") - she thinks it's just kind of an annoying affectation.