I was using the official BT client for a while, but was not happy. I gave Transmission a try.

The ability to throttle upload and download bandwidth is perfect! The ability to pause/resume all downloads at once is great. One thing I miss: If I open a gazillion torrent files, Transmission attempts to download them all. The official client only starts 4 downloads.

One great thing that the official client messes up: Temporary files are not left lying around. I got burned with BT leaving huge files (several gig each) in ~/Library/Application Support/BitTorrent. Transmission doesn't do that.