said by Matt
:I don't rent movies from the XBM because my XBox doesn't have a HD, but my understanding is you CANNOT watch them as they are downloading. You have to download them 100% before watching. I've only spoken to one person (of dubious technical knowledge at that) who was complaining about it. But based on the behavior of smaller downloads I've done (game previews, movie trailers) I would say he was right, you have to wait until the download is complete.
If it takes less time to download a movie than it takes to go to the video store then it is worth it. Less gas to waste and not having to get dressed and find parking equals a win situation.