Newsfire and Acquisition creator Dave Wantanabe is currently working on a BitTorrent client for OS X dubbed Xtorrent. As with all of Dave's creations, Xtorrent looks amazing. Try out the public beta for yourself. While it does look great, feature-wise I don't think it's any competition to Azureus, performance-wise it's no Transmission and cost-wise, it won't be free when it's released. Update: Performance should be comparable to Transmission.