been using my iPhone with Gaia GPS Offline Topo Maps for the past few years. I have my phone with me on all trips anyway and it actually has a great GPS receiver, so figure why carry another. map, compass, watch altimeter are backup.
Battery life if the only issue (if the radio is left on). But if you keep it in airplane mode until you need to make a waypoint or find position, the battery will last for days.
i'd imagine all modern smartphones are pretty much the same.