GitX is good (and free), but if you're looking for the best Git client for Mac, without a doubt it's Tower ( http://www.git-tower.com/ ).
I've been using GitX for months and it's pretty good, but it still buggy at times. I've been using the GitX (L) fork, since the original GitX doesn't seem to be actively maintained anymore. It's available here: http://gitx.laullon.com/ A few days ago a co-worker turned me on to SourceTree ( http://www.sourcetreeapp.com/ ), which is also free and developed by Atlassian. I've got to say it's a bit more polished and so…
Also, it's great that Source Tree supports multiple projects. I tried Tower a while ago and it only allowed you to have one project open at a time. Maybe that's changed now, but it's also not free.