An interesting feature has appeared in the last build of μTorrent (build 17618): preview of partially downloaded video. If you click on the Stream button after starting to download a torrent, then after a few seconds or minutes it becomes green - and then you can start watching.
It turns out that torrents can now work as YouTube: you can start watching a movie almost immediately, after a little caching. If the movie's bitrate is approximately equal to the bandwidth of the channel, then just in time for the end of the viewing, the file will be downloaded. ')
According to the first testers, the feature works fine on torrents with a large number of seeders. The client is optimized for watching movies in DivX format, and with some other formats there may be problems.