Pretty much everyone uses it in London. I've continued using it in Madrid with great success. It's generally very, very good in the cities where it is available.

This is a Google Maps complement, not a replacement, as Citymapper only does navigation within a city, so you can't plan inter-city travel. Also it's optimized for A -> B navigation, not so much for "city discovery", travel research, or more complex itineraries, for which I keep using Google Maps.