So there are three families of tools:

* STREET and cities by car, bicycle and even on foot: iGO, TomTom etc

* Outdoors/hiking (no network): Locus, Osmand etc. Totally offline with free OSM maps.

* Sports (activity logging): Runkeeper, Sportypal etc. Most of these don't need a network during the activity and after uploading at home you can see more with maps etc ususally on the PC.

In my opinion todays phone's are fully fit GPS wise. But be sure to have a backup when hiking (friends phone, extra phone, paper map).

OSM maps may lack a roundabout here or there but (in Europe) they show an awful lot of even (ancient) hiking trails. And you can find many, many tracks in .gpx format for easy guidance in Locus etc.

So, define your goal and take your pick.