I'm using the Git Tower beta and I have to say, it works like a charm. It does everything that GitX does, but provides a better UI for everything.

The only thing I miss from GitX is the ability to discard changes per changed line/block. In Tower, it is possible to stage per changed block though.

[edit] One thing that I forgot to mention is that the developers are very active and responsive, even though it's a closed source project. I made a few suggestions and got replies for each of them within a week. (Compared to Gitbox, Git Tower is built by a whole team of devs.)