I have MM on my laptop so getting it for my iPhone was a no-brainer. I haven’t investigated the cost of buying tiles as you need them, but I suspect it might be pricey. I think other options (viewranger?) might prove cheaper if its just for your phone.
It is a VERY useful app for navigation (I try not to rely on it solely, but only because I’m a trainee ML) and can really help improve the flow of a ride. I tend not to use it for route tracking, but that’s just because I prefer the interface of endomondo, and I get annoyed having to remove the red lines it leaves all over my maps.