I use an iPhone for GPS stuff.
Have been using MotionX GPS ($1.99), which is supposed to load terrain maps in from OCM, but guess it's importing tiles from an old server or tiles that are rendered slightly differently, as the maps it's showing aren't the new bilingual version.
EDIT: I was wrong - MotionX is using the bilingual maps, but I had to delete everything and re-download, rather than just update to get the new maps.
Just testing Gaia GPS ($19.99) and Maps 3D GPS ($3.99) - both use OCM. Gaia looks OK but is a bit expensive, Maps 3D GPS seems like it'll do what I need it to do - and offers the 3D terrain map view as a useful bonus - for a cheaper price. Will try it out....
EDIT 2: Sticking with MotionX. I like the '3D' of Maps 3D, but you can't organise Waypoints and Tracks so well - and it misses out on a few other features.