Count me in as the first vote against Transmission.
It's great for lightweight torrenting (one or two torrents and lots of apathy. seriously.), but anything beyond that and Mainline/Transmission/TomatoTorrent go out the window. Bits on Wheels is sorta good if you're a PPC user, otherwise it uses just as much resources and isn't as powerful as the Azureus unibin.
One caveat: some sites ban a lot of the Mac clients because they don't follow the protocol correctly (cough Transmission, BoW) or is just too obscure (Tomato Torrent), so check it out before you start using a new client.
For guaranteed compatibility and tons of features, go Azureus.
Otherwise...
Note: I'm totally biased, as I bittorrent all the damn time and can't stand the utterly pathetic features in every single client outside of Azureus when it comes to banning peers that send bad data, having complex seeding rules instead of simply ratio/time based rules, actual queues, global and per-torrent upload/download speed regulation, manual announces...
Azureus is the only one that really works properly with multiple torrents when it comes to connectability (read: NONE of the other Mac clients will have connectable torrents if you start opening torrents halfway into your session, even though the ones you opened at the beginning are connectable, hence requiring a restart every time I open a new torrent to make it connectable?!? Hell no, I don't think so.), and best of all, is the only one that actually only connects to peers from the tracker and nobody else if you want it to.
In addition you can force hash checks anytime you want, which surprisingly enough some clients can't do even though they can all fast-resume (no hash checking) torrents in their own way. The best some of them can do is hash check automatically after finishing a download, which obviously leads to corrupted files, misreported statistics, getting banned from peers because you downloaded and then proceeded to upload a file that got corrupted and stuff like that.