I'd heard recommendations for glGo, but it is compiled for PowerPC and no longer runs on MacOS if you upgrade to Lion (which removed PowerPC emulation). So that's out, as are many of the older compiled clients.
Goban.app runs fine, but I just lost a rated game because of a brief network connection failure. All my other network clients recovered fine; Goban dropped the connection and didn't alert me, so the game timed out. Likewise, the Java WebStart app has crashed repeatedly for me.
I love the Tetsuki iPad/iPhone app, but it crashes whenever the chat function is used for me. Watching a highly rated game with lots of kibbutzing is a sure way to get it to crash. Again, this lost me a rated game recently.
I would like to lose because of my own limitations or mistakes, not the client's.
Thanks!