I'm a big fan of music documentaries of any kind... even if it's bands I'm not into, I will still watch it. Whether it's DiG!, Drive Well, Sleep Carefully: On the Road with Death Cab for Cutie, I Am Trying To Break Your Heart or anything else... I will usually watch it... even if it's about The Beatles.
All that being said, I'm pretty pumped about the documentary over Motorhead's Lemmy Kilmister. I only know "Ace of Spades," so, I'm not a hard core fan, but, Lemmy is a pretty interesting dude.