Some good, some bad. The bands tend to be great as themselves. REM comes to mind, as does the Homerpalooza episode. Remember Lisa Kudrow as Lisa's best friend? Sometimes a mediocre actor can become a good voiceover.
Has anyone ever noticed that most of the guest stars get to say Yoink? LOL I love that line, Mel said it, Mark McGuire..of course those are the only two I can think of right now (D'oh!), but there were others :) Have a Duff Day :)