I agree with being at the point where I'd happily pay for it (I'd actually pay for a mobile and faster desktop version that sync, as it's a bit sluggish on my netbook). What Joel said about it being a list of lists (a very simple idea), is what I like so much about it. Currently I'm using it for standard simple use cases (keeping track of stuff I need to do), non-standard use cases (tracking my weightlifting progress, between each set), and collaboration (my landlord and I keep a list of things to do). Oddly enough, I don't use it for software at all (what I investigated it for in the first place).
I introduced my (non-technical) girlfriend to it, and after a short explanation of how I used it, she took to it immediately, and uses it daily to keep track of her to-do list.
I introduced my (non-technical) girlfriend to it, and after a short explanation of how I used it, she took to it immediately, and uses it daily to keep track of her to-do list.