I've only used the Android version, but
Navmii has stood me in good stead when abroad with no data. It's more a car satnav like system (i.e. not useful for following a pre-planned track), but is useful for working out where the hell you are, and how to get to the next known place on the route sheet.
I've used Viewranger too in the past, but abandoned it in favour of Navmii for random acts of navigation, and OSMAnd (
which is, of course, no use to an iPhone user -
ETA: correction - it does appear to be avilable for iPhone - I always thought the "And" bit was "Android") for walking and following GPX tracks.