I have been using Habitica for quite some time now and i must say, it has been the most useful in helping me establish a structure to my day. HOWEVER, the To-Dos are very limited in its functions. It's hard to implement a full task management (such as GTD methodology). It pretty much only supports next action steps, not any layers such as Projects or Waiting For. Also, syncing across devices can sometimes be problematic I hope they will improve the To-Dos and make syncing smoother.