BitTorrent worked fine for me but as others noted, it grabs a little bit too much CPU time from my computer. It has some nice options that other clients don’t have but I found out, that I don’t really need them. I also have seen some interface problems with the old version (the bar at the bottom of the window loses it’s skin).
I have also tried Transmission and the new version is really a pleasure to use. It’s faster than BitTorrent, it uses very less CPU (3-5% on a 1.5 Ghz machine) and has a little memory footprint. Less options, better performance, nice interface and it’s free. Maybe you’ll give it a try.
So I’d like to thank the developer for creating this very usable client for the Mac, but at the moment, the competition seems to be one step ahead.