How well does that work when a video isn't very popular though? The value of a thing like youtube is that anybody can put a video on there and it will be available for anybody else to play, instantly, for the foreseeable future.

I imagine the non-popular videos are sent directly from central servers. By offloading the most popular spikes to P2P maybe they're able to reduce costs enough to make it worth it.

Interesting, it doesn't seem to work here because my employer's VPN blocks the torrents.

To add to that, there's also the fact that any user of any ISP that throttles/blocks torrent traffic is probably going to have the same experience. Plus the fact that there's at least two people I know whose ISPs actually cut users' down speed by double of any up speed (in order to "passively" cut down torrenting without actually restricting it). So PeerTube becomes less a YouTube replacement and more an interesting implementation of streaming video via Bittorrent (which some clients already support).