I used the map system on my phone pretty extensively last season in areas that have little to no coverage and thought it worked great. It took a little planning ahead of time to make sure I had all of the maps downloaded in the right format and scale, but once the maps are downloaded it worked pretty flawlessly, The downloaded maps load quicker and require less battery than even using the phone when I have strong WI FI signal.
I used the phone based maps for a three day backcountry hunt were I had absolutely no coverage at all and had no problems. I use a small external battery for my phone that gives three charges and if my phone and external battery are fully charged I would have enough juice for four to five days. Given that my phone screen is so much better than any GPS screen and I carry it with my anyway for pictures, I cant imagine ever using a stand alone GPS again.
