I have used the OnXMaps and The Trimple GPS HuntFish now EOL. Both worked great. I just turn off cellular service but i dont go to into airplane mode as others cause i have noticed at least on my phones it does not track you with GPS, i have new iPhone 6 so maybe airplane mode works now.. ill have to test that. I have used the phone right next to multiple garmin GPS units before, and as long as i have downloaded the maps prior to being up in the backcountry it has worked so much better then my big clunky GPS. I have even been on hunts when originally testing the capabilities of the apps and the phone gps vs the standard Garmin and had areas where the Garmin wasnt picking up my location due to satellite interference. I wanna try Back Country Nav but i havent seen a reason to switch as OnX does a good job and gives me boundaries and so forth which is helpful, plus i can switch from topo to aerial too.
I carry a battery backup from Goal One with me, does about 5 charges before its drained. I used to use the Mophie Juice pack but that thing drains the better quicker and for some reason my iphone rejects it like a bad implant sometimes, and in those cases your screwed. Did it do me the other day when i was way back scouting, i had no battery and the juice pack stopped working. I was pretty screwed, and totally missed my destination in the timber.