I have been using a $12 app called "Back Country Navigator" and I really like it! It can pull in KML or GPX files, and has many different topo and satellite imagery layers to choose from; and will pre-download map data.
I figured "I have a $600 phone, why buy a $600 GPS? It's just application data". A waterproof Samsung S5 in an Otterbox Defender helps, though, it's a tough phone. I've seen them fall from 200ft towers and survive... though it didn't survive my wreck back in June! Good thing they're a dime a dozen for used devices