I just made the jump from todo.txt to Workflowy and paid for pro service within a week. Project hierarchies really seal the deal for me, while todo.txt has a flat layout, even if you organize items by project or context.
I like the idea of Workflowy, but it really needs offline support. I can't even realistically evaluate it without that. Last I heard they were working on it.
Just used Workflowy.
THANK YOU!
This is what I've been looking for. I have lists on springpad, on Tasks on Android, on IMDB, on a text file, etc.
This changes everything.
Right now, I can't use it for TODO, and use it only for brainstorming. For TODO, I now use getflow.com