The Stockfish team has made tremendous progress recently, thanks to 1) putting the code on Github and accepting patches, and 2) building an open, distributed testing framework that tests every patch by playing thousands of games.

http://tests.stockfishchess.org/tests

OMG, that is cool! Testing by playing 1000's of games, like QuickCheck for chess.

By the way, matches are currently ongoing to determine if Stockfish should be ranked #1 or not.

http://tcec.chessdom.com/live.php

What's the best open source GUI for it?

Arena and Lucas chess are good. I like Lucas for it's simplicity though. And it is fun for my kid to play with. It comes with a tutor option also.

http://lucaschess.pusku.com/

http://www.playwitharena.com/?Download

Edit: Not sure if all of these are Open source though. But they are free.

Try 'Scid vs PC' or ChessX. You'll get a good-looking GUI, plus some cool database features.

http://scidvspc.sourceforge.net/

http://chessx.sourceforge.net/

A cool feature people may not know about - many of these GUIs allow you to play two engines against each other. It's fun to watch two strong programs slug it out. :)

As far as I was able to tell, when I was interested in chess a few months ago, there is no acceptable Mac GUI for Stockfish.

It also works with GNOME Chess.

Google "Scid vs Mac"