A documentary is good if it makes you think, or increases your interest in a certain topic. A good documentary is anything but boring. Take Riding Giants, for example. I've never had an interest in surfing, but after watching that movie I was ready to ditch my job, buy a board, and move to Hawaii.
Anyway, last night I watched a documentary called Spellbound. It's not very new, and the subject matter sounds...boring. It's basically a documentary about 8 children who were in the 1999 National Spelling Bee.
It's a great show, and while I have always hated spelling bees, and can't spell to save my life, the show was incredibly intense. Each time one of these kids got up to spell I found myself gripping my pillow (I was in bed at the time), hoping they survived to the next round.
I think the most endearing kid is the boy who starts the movie. He is trying to spell 'banns'. The anguish on his face, his attempts at spelling it...priceless.
Anyway, if you're looking for a good documentary, I highly recommend it.