Azureus is amazing when it comes to control, but possibly somewhat overcomplicated if you just like to start downloads and couldn't care less about the rest - I've found it takes a fair whack of system resources somehow.
Utorrent, well, it just rocks. Very small, feature rich but not too much so for those that want to use such features, but there's no real need. very easy to use. The downside is that it has no write cache of its own, meaning that constant use (IE 24/7) might just wear out your hard disk sooner - I've yet to have this confirmed, but I've heared people saying it, and their explanations sound pretty convincing.
BitComet - hmm - recently controversial because version 6.0 ignored the private flags. There seems to be a lot of people using this version bitcomet - possibly because ignoring private flags is advantageous on private trackers for convoluted but never the less important reasons which I shant go into. Sufficed to say that ignoring private flags is cheating

. Other than that I know little about this client.
Bittornado - kinda like the bittorrent original client, but better. Not a bad one.
Also, I very much aree with the post previous to mine.