I’ve only played with this for a few minutes, but so far it seems to have an excellent, simple, clear GUI with the core features I’d expect. Way better than any other Git client I’ve seen for Mac. And it’s free!
GitHub for Mac v1.2.1 (NB: works with any Git server, not just GitHub.com!)
I’ve noticed some cosmetic bugs – e.g. it renders all the user-account avatars completely wrong (puts wrong image next to each commiter) – so I’d advise “use this with caution” until you’re sure it’s got no fatal bugs. Although, since it’s from GitHub.com, I expect it’s pretty robust.