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.
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.
One thing it’s bad at (ironically because it was made in London) is the data model for Tube stations, it seems to think a Tube station is a point, whereas they often sprawl underground over large distances. It doesn’t have much of a concept that you can be walking for 10 minutes to connect between lines, and will make dodgy suggestions on that basis.
I don’t know how they undercut TfL on the price but I presume they’re doing something tricky with redistributing monthly or annual passes.
They still have consistent problems with randomly charging you random amounts on top of the weekly cost. It's a normal MasterCard so you can check it on the TfL website, but I've had to work with them a few times to get refunded from random extra charges (usually 0.50 to sometimes 7 quid) which they can't explain why they happen.
The official app from the public transport network was revamped last month, and now copies a lot the UX of Citymapper[1], albeit with worse performance. The app use some weird caching system, and sometimes, even after refreshing twice, you don't get the correct timetable, and it's especially bad at reporting traffic alerts. Citymapper never failed me like this.
[1]: https://play.google.com/store/apps/details?id=com.micropole....
I am extremely extremely impressed by how absolutely perfect the UX of this app is. I don't know who designed it, but congrats to them. If you haven't used it yet, you must.
Last week, I landed in Brussels and needed to catch the train. Google Map's best route was a 40 minute wait for the next train. I opened Citymapper and it suggested a train that, although timetabled for 5 minutes ago, was actually delayed by 10 minutes.
The data coming into Citymapper seems to be much more reliable. Google Maps is a great general tool that works anywhere, but Citymapper puts great attention to work perfectly within cities.
I still resort to instructional YouTube videos + ticket machines for tickets, though.
Multiple transit options with real-time info, delays, prices, and solid offline support. Shows exactly where you are on your trip in terms of signs and stops. Also has little details like which exit facing which direction to leave when getting out at a station which makes all the difference.