This weekend I started using the Bittorrent P2P network, with the Azureus client. I choose that client (there are a lot more clients, with more or with less functionality) because on several forums I read that it had the most functionality.
I must say, without having tested others, that the program is easy to use, looks good, has tons of features, and seems very stable. One of the most wanted features in a P2P client is the ability to limit available bandwidth. Azureus can limit the bandwidth in general, but also per torrent you make available. So you can limit popular (scarce availibity) downloads to a certain amount of kbyte/sec and leave the rest on umlimited (that obeys the connection-specification you entered, so 512kbyte/sec upload max if you said that your line is that fast).
What I need to get used to, is the inability to search for torrents via the client. But that’s the nature of Bittorrent: torrents are not stored on a central server. You don’t even logon on a central server.