I've read a part of the book about SNL's history and this documentary is kind of like a companion piece that just focuses on the '80s. It's not thorough, but a casual SNL fan like myself won't notice what they've left out. I think you do have to have some familiarity with the 80's casts to enjoy the documentary though. Otherwise there would be all too many unfamiliar faces.
Though there are many clips of sketches included, the documentary is mostly about former cast and crew dishing about the show's troubles in the 80's, including the behind-the-scenes politics and the legendary firing of almost the entire cast from one season to the next. It's pretty interesting if you're fan enough to watch it, but if you just want to watch sketches, you'd be better off with another of the multitude of SNL DVDs.