I'm finding that I'm making plenty of use from Amazon streaming video to justify the $80 I paid for prime. In general I don't care that much how long it takes for packages to get to me, but 2 day shipping is undeniably nice.
I'm watching movies that I should have watched years ago, and there are hundreds of documentaries - NOVA before the current season, TONS of David Attenborough stuff, not to mention a lot of animated stuff.
There are a lot of movies it doesn't come with for free, but there's never nothing worth watching.
Also, the Roku box is probably the best $70 piece of hardware I've ever bought. For such a cheap little hockey puck, it's full of fun.