Transmission is an awesome application. It's much lighter weight than Azureus and does all that I need it to do. In fact, Azureus seems to slow down my computer when using it and has a much higher network usage. I notice it, because I play networked games when downloading stuff and there is no performance hit with Transmission.
BitTorrent isn't just used for trading warez -.-, Blizzard hosts the Warcraft Client installer and patches via Bittorrent. I don't like the Blizzard downloader since I can't control the bandwidth it sucks up. But if I use a torrent client, I can better manage my bandwidth.