I've been using HabitRPG for a couple of months and I don't have freakishly great self-motivation (although I do have a friend who's in my party, so there is some peer pressure/accountability to keep me on track). So far I've found that it's good for habit formation for the kinds of things that are more about remembering to do them than about negative motivation. It's also reasonably good in a GTD sort of way, in the sense that I'll put a big task on my to-do list, and then when it comes time to actually start on the task I break it up into much smaller subtasks to provide myself with incremental rewards towards completion.