I just finished watching it and I did enjoy it a lot. There were lots of things they could have done better but I still highly recommend watching it.

The biggest area they neglected was the acoustic guitar and it's role. Bluegrass is responsible the acoustic guitar moving from simply being a support instrument to a fixture as a lead instrument. As a result, guitar pioneers like Clarence White got no mention at all and even Tony Rice is barely mentioned, and then only as a member of some bands.

Tony is easily responsible for more Martin D28's being sold than anyone on the planet, (pure speculation on my part. LOL).

Still a must see if you're interested in the genre or want to learn more.