I’ve not used it myself but recall Brett Terpstra waxing lyrical about it a while back. I think he may have mentioned it on his most recent MPU appearance too.
It does seem like a great idea and I’ve watched some of the videos on it. The fact that you can just leave everything where it is and not have to move things in to a “bucket” app is great. Yojimbo and Circus Ponies’ Notebook proved to be my graveyards of best intentions with that type of app many moons ago.