In the time it takes you to tell someone "Remind me to lend you that book," you can type a quick note in Due — way faster than creating an event in a calendar or more elaborate to-do apps. At the prescribed time, you'll get a notification reminding you to complete your task.
WIRED Ease of use means you'll actually use it — a lot. No superfluous junk like tags or lists to deal with.
TIRED No integration with other calendars or organizers (though if you want that, you're kinda missing the point).