I watched and waited for many years for the available technology to be good enough to make it worthwhile. It started to arrive, in my opinion, just about three years ago. Now it's very much available and really it was the only reason I paid the money for a 'posh' phone. Until then I had bought the cheapest that did the job of just being a phone.
The catches with using a mobile phone are waterproofing, ruggedness, battery life and outdoor screen readability. Those are the reasons why I bought and still use the Montana quite often as well as the phone. Of course, the phone is with me anyway.
My Sony is waterproof and a good rugged case is available for it. The screens are certainly better now than they were a couple of years ago but a proper daylight-readable screen would be better. They are supposedly in the pipeline now at affordable prices.
Making your own maps is a great way to save money, and there are some good and free trail maps online that can be converted. I bought my Montana without maps and made them myself. 2D maps are easy to make for Garmin and Android phone software; it just takes a little time to do. Even 3D is possible but that's out of my comfort zone and I don't need it anyway; I can read contour lines. That said, it is fascinating doing a 3D fly-through of a route on the 3D maps that I have, especially on a big monitor on the PC.
I'm happy to help out with any map-making advice, by the way. Not that I'm an expert, lol.
The best Android software I've found for (offline) outdoors use is AlpineQuest. It works online or offline with either cached maps that you've saved in the app, or created ones like mine.