Bah, just wrote a short description of the different clients I've used and posted in the 'torrnet' thread in 'Technology', and I don't want to paste it here or rewrite it, so I'll just make it short:
My recomendations:
µTorrent <-- Windows
rTorrent <-- *nix / POSIX (if using -private- trackers)
Azureus <-- *nix / POSIX (if using -public- trackers)
More explanation on the clients in the other thread.
Otherwise this is a quite good summery of the most used BT clients out there:
http://en.wikipedia.org/wiki/Comparison ... t_software
Paranoid people live longer.