Updated tutorial with better torrent client here.
Most large torrent files (8-20 gb) take quite a while to download. Is there software for Mac OS X that allows me to watch them while downloading?
I tried all (I mean all) torrent clients for OS X that I could find.
There were people before me who wanted the same feature and have asked the most popular clients (uTorrent and Transmission) whether they would support serial downloading. Developers replied that that would defeat the purpose of torrent protocol (it would seem that uTorrent changed their minds and have such a feature for windows).
Finally I found one that could download file parts in order. It's called BitThief. It's a research project and has an awful interface. But it works, and most importantly downloads serially.
-> -> ->
After the file is put for downloading, the speed must be good enough for watching. For example, if the movie is 1:30h long, I check the ETA time to be less than that.
To watch the movie I use MplayerX (an OS X GUI for mplayer). It can be downloaded for free from OS X App Store. Mplayer is light, can play EVERYTHING, and navigation is done with the arrow keys. Mplayer automatically regenerates the index (the thing that allows using the progress bar). The OS X gui is awesome and supports the Lion's full-screen view.
There's always a chance that things could go wrong - low speed or high speed in the beginning and then low speed (which is even worse).
Keep in mind that in order to watch a 1:30h long movie that is 20gbytes, you have to download with ~3.7mbytes/sec.
Update: tested with 700mb AVIs, 2gb, 6gb, 8gb, 20gb MKVs (was lazy to check the codec, h264 mostly I guess).
Update 2012.07.02: This solution has been serving me quite good. Still here are the issues: BitThief is not very optimized and while other torrent clients can utilize my whole bandwidth, BitThief fails from time to time, probably because it is downloading sequentially. Also magnet links are not supported and the interface sucks so much.
Update 2012.08.16: A fix for utilizing magnet links: magnet->torrent converter. Utilizing magnet links allows this process to work for thepiratebay.org and other sites utilizing the new magnet url technology.
Update 2013.02.04: The newest version of BitThief now supports Magnet URIs. They're saying that a new look and feel is also coming.