I'll add my opinion that, overall, the series was quite good.
I have a few criticisms but, as Ebbie says, no way he could have covered it all. The things that I would have like included, or parts I'd have liked shortened, would have made it a better series for me, but not necessarily a better series per se.
As my wife pointed out, for those less familiar with the historical roots of what we know today as country music, the series held a few
revelations. Black influence on country music was an especially
important point to make. The white Delmore Brothers on-the-road solidarity with the black musician, DeFord Bailey, was something that I
hadn't heard of before and was glad it was included.
So, all in all, I think a job well done.

Oh yes, thought it was interesting that Ken Burns chose to end his look at country music with the death of Johnny Cash.