I have been playing with this feature for a few weeks now, and the potential is huge. It learns from your inputs, and it still needs some work, but the basic idea is very solid.
Nothing from any app I've had open has been added to Timely from the Memory on either my (a) work macbook (b) personal macbook or (c) surface. I thought it could replace rescuetime and allow me to integrate that with my projects but it doesn't actually work. It didn't work for any of my devices.