Transmission has a *well* documented bug that plagues a certain group of people it seems. Transmission will hang, beach ball, start working, then hang again, and repeat. Not sure what it is, but many people acknowledge that this happens, and it even happened to me constantly on my 15" MacBook Pro and MacBook Air. Fresh OS reinstalls, etc. Nothing could make transmission not choke.
But having said that, transmission is still great *if* it runs well on your system. Azureus is in second, and to me uTorrent comes in 3rd.
Lastly there is always the Opera browser that has torrenting built in.
Edit: I see a lot of votes for uTorrent, did they finally add the ability to select which files in the torrent you want to download? In the betas I tried this functionality was not in place, and it forced me to download every file even if I only wanted a few files from the torrent.