I'm a big TiVo fan, have been for years. When I received an invite for $15 credit to try out Amazon Unbox on my TiVo I jumped at the chance.
So far I've rented the past tv episode of 30 Days - Outsourcing, the movies World Trade Center and Miami Vice. Just ordered Catch a Fire and Fast Food Nation.
One downside is that your rental is only good for 24 hours, start playing your rental and you've got 24 hours to watch it as many times as you want.
You get full DVD quality downloaded to your TiVo box, the World Trade Center movie was a 2.72 gigabytes.
I haven't purchased a video yet, but apparently there is an online media center at Amazon were you can re-download it at your pleasure.
This is a great service, still trying to determine whether on-demand cable is better, same, or worse.