Azureus is relatively ugly (though MUCH better in the last release or two) and rather complicated, but it's also immensely powerful and handles torrents in just about any language or character set without issue (I occasionally use it for Japanese videos, hence the personal concern).
I tried the standard client and Tomato, but although they're very easy to use they choke badly on some torrents (particularly those with oddball characters in filenames) and they have little customizablility (ports, caps, seeding rules, sorting systems for multiple simultaneous downloads, etc).
This new Bits on Wheels looks like a more Mac-savvy Azureus, so I'm intrigued, but I'm comfortable and happy enough with Azureus (which also has the advantage of being open source) at this point that I'll give it some time to mature before I give it a shot.
Question on BoW: The features and interface look almost exactly like a Mac-ized version of Azureus, so I wonder if it doesn't use the same core codebase. In fact, there was a beta version of Az recently that had a visual representation of the swarm that looked almost exactly like their graphic, making the connection even stronger.
Of course, if it does share the same codebase or some of it, that's probably a good thing, since Az seems to be a pretty solid chunk of software hindered only by the fact that it's Java-based.