I use Citymapper extensively on trips to cities it covers.

It's at its best in London (where, IIRC, the dev team is based): there, it's a life-saver. It's also effective in other first-rank anglophone world cities, such as New York. I've found its coverage can be patchy/incomplete in some cities, presumably because the data sources they aggregate are incomplete or inaccurate: at its worst, it degrades gracefully to the state of an offline maps app with a public transit route-finder.

It has been around for some years and they used to integrate with Lyft: I think the idea is to tie into transit pass apps as a way to monetize their service.