Great list. I'd like to mention I use Tower app for a Git GUI. I especially like that it shows a diff right as you're looking at changes and lets you stage chunks of code which I find very helpful.
I've used Gitx, SourceTree, and Tower each for more than a week exclusively. So far, Tower by far comes the closest in most features. One key feature for me was being able to easily switch between commit-list view and file-browse view (the latter to get to a file and view its history). I haven't tried Gitbox, because based on their feature list, it doesn't even come close to SourceTree, which in turn had less features than Tower.
One of my biggest problems is that all of these programs seem to emphasize pushing, pulling, merging, rebasing, etc. But I don't want any of that stuff, the command line is awesome for all that. I just want to search, browse, and compare commits a bit more easily.
There are a few small things Tower is still missing, but the Tower team have been really responsive and receptive to implementing those changes when I talked to them (which I think is a benefit of actually having to pay for the app).
I've actually tried Tower but I prefer the simplicity of Gitbox's UI.
If you like Tower, you should check out SourceTree (http://www.atlassian.com/software/sourcetree/overview). It's free from the Bitbucket people.