It seems like most people are wild about Azureus and some people are really hating on Tomato Torrent.
Azureus is cool, until you realize that (for most people), the purpose of a bittorrent client is not to sit there and stare at all the changing numbers and graphs. It's bloated as hell, and while it has nice features, it clogs up your computer quite readily. Nice idea, too much bloat (for my taste). It's an awesome program if you enjoy sitting and watching your torrents download.
Tomato torrent is as unbloated as they get. It's lighter on the CPU than the legit BitTorrent client (I've tested it), and it's very sleek and pared down, with a simple aqua interface. On good torrents it will quickly max out my connection (300kb/s or so). If you download in the background go Tomato.