The weakest episode is the Dolly episode. It's a bit like a Louis Theroux episode, your left a little uncomfortable seeing into her less than normal World, a bit like a view of Michael Jackson or Elton John's life. But her talent is indisputable.
Apart from this, the other episodes are magnificent. There is so much cross over between Brian and his guest in the music that influenced them. Watching Mark Knopfler and he trying to sing in a duet a song neither has probably heard or sang for decades, filling in the blanks of recollection for each other, was wonderful.
I hope there is a 3rd season and that he takes the time to spend a day with two people in one episode rather than one, with Alex Lifeson and Geddy Lee as they come as a package and with ACDC having toured the US (before Johnson) with them.
Also, would be great to see the Edge, maybe Ozzy or Tony, and definitely David Coverdale. John Lord from Deep Purple, or there's also gene Simmons from Kiss, Steve Howe or Rick Wakefield from Yes. There's ample room for new episodes.