I haven't used Xtorrent but I did go through a whole bunch of torrent apps before finding a satisfactory one.
Transmission is terribly slow and doesn't register many seeds (I once had the exact same torrent going in both Transmission and another program - possibly Azureus, but I don't remember right now - and Transmission had about 50 seeds connected while the other had upwards of a thousand). I've read that it's because Transmission doesn't play nice and is banned from a bunch of trackers.
At the moment I'm using BitRocket. It's still a beta and somewhat unstable but it is much faster than either Transmission or Azureus. It mostly doesn't crash anymore if my connection stays live and if I don't try launching it with active torrents (as in, I pause the torrents when quitting then resume after launching).
I also don't really like the idea of just searching for torrents from within the client since I prefer to get my torrents from a trusted site such as Mininova where people complain when someone posts a fake.