I've set it up for GTD, but don't currently use it because there's no good way to use Siri with it. I use a fairly simple implementation now that can be used with Things, Omnifocus, Wunderlist, Toodledo and others. I have a preferred order for my lists, with contexts first and project list, waiting for, tickler and someday/maybe following. I use the tickler list for start dates if the list tool doesn't support them, as is the case with Wunderlist. I use the star as an indication of priority/focus/intention but of course it's good not to go overboard with that. This is a very fast, simple implementation that gets out of your way.