I tried using it on OSX 10.8.1 but I can't seem to get it to work. It doesn't let me drop anything into the box where it says "Drag Apps Here To Create New Group"? It just does the typical "delete from dock" poof animation.
Lots of people are also suggesting hiding the other apps. I might try to implement this as an option, but what I'm going for is de-cluttering the command-tab app switcher, and I don't think hiding the apps will achieve that. However, this feedback is inspiring me to see if it's possible to hack the task list, or whatever it's called.