Tough to say ...
I just downloaded this on my iPhone to take a look. After reading their getting started guide, I can see how the app could be really useful for some people. But it's one of these programs that requires a real commitment. Essentially, you've got to create a new habit of inputting every single thought you have about something new you'd like to do or should do, followed by regularly taking out time to organize those thoughts in the software into proper categories.
If you're disciplined enough to use it constantly? Then yeah, I can see why you'd pay the money for it. (EG. You shell out the $10 for it on the iPhone and start using it, but quickly realize you need access to all of this data, and ability to immediately input more, even while you're at your desktop Mac. So then the $50 is cost-justified, because it's a small price to pay to keep your life this organized.)
The problem I have with it is that it competes with a lot of other apps out there for my time and energy. For example, I used to use Evernote to keep track of my random notes and thoughts. It didn't really have the ability to pop up reminders based on deadlines - but it still worked well as a place to store and organize a collection of info, and cost nothing to see it in multiple places (Mac vs. iOS devices vs. Windows PC).
People using Apple's own Reminders app can do *some* of this, too, without spending a dime. It just won't allow data input in the same organizational system they came up with for "Things".
Dang, that was expensive before, $10 for phone, $20 for iPad and $50 for desktop.
(still will cost $35 for desktop)
Is it really that good?
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Yes