I started my review here on this documentary a long time ago and failed to finish it for some reason.
Anyhow, this will be a surprising entry here, probably, but this "rockumentary" is on Netflix right now and it's worth watching.
I suppose I should qualify that by saying it's worth watching if you like ZZ Top. But maybe it's worth watching even if you don't. Indeed, I sort of like the Clash, but there's a rockumentary out there on them that's really good, even if I can't recall its name.
Anyhow, this look at ZZ Top, filmed before the recent death of one of its members is a nice, and fannish, look at the band, it's origins and where it was just prior to the noted death. It touches on their rise as a Southern Rock/Blues band into a rock band, including a period of time in which they sat out for a while and why they did so.
It's a nice look at the band, and better, frankly, than some documentaries of this type. Worth watching.