After a few years of using Azureus (or Vuze now) without major problems other than kind of large CPU and especially RAM usage, a friend suggested I give µTorrent a try since they now have Mac builds. I did, and after the first two builds which suffered from a CPU hogging bug, was surprised to find version 0.9.1.2 running smoothly for a couple of days, using about 2-5% CPU and 22MB of RAM with DHT, Peer Exchange and outgoing encryption enabled (before switching these on, it was taking about 0.6% CPU!).
That's pretty impressive IMO - previous BT clients I'd tried (Tomato, Transmission and the built in one in the Opera web browser) either seemed a bit cut-down and/or just didn't seem to download at the same speeds Azureus had achieved. So far µTorrent has worked very quickly, and has the further benefit for me of reliably working with pasted torrent URLs - Azureus almost always times out when downloading .torrent files via HTTP (no idea why; maybe a Java thing, but it happened on both my Mac and Linux boxes).
The only cons so far are that it's not open source, and that there's no Linux build... but I don't use my Linux box these days anyway. Give it a try if you're exploring BT clients on the Mac!