Comscore, whose accuracy is generally between a Lotto Quick Pick and a drunken dart throw, says Google Maps usage has fallen since Apple Maps came on the scene. The Guardian has a good overview: How Google lost when everyone thought it had won.
We shouldn’t be surprised that in the absence of choice, people take the path of least resistance. What’s missing in these discussions is how it’s criminal Apple gets away with not allowing alternative defaults for maps, browsers, calendars, and any number of other areas, which means every time you click a link or address in the OS it opens Safari or Apple Maps, in my opinion inferior apps. Some developers get away with this by having settings to set Chrome or Google Maps as your default, like Tripit just added, but this is implemented in a hacky, per-application way, and every app puts their setting in a different place if they support it at all.
If Microsoft did this a decade ago we’d call for the DoJ to reopen their investigation. Apple has the best phone, best tablet, and in many ways the best operating system — we should not give them a pass for this blatantly self-interested and user-hostile stance. Defaults matter.