iPad mapping application with custom maps

I've had pretty good experiences playing around with iGIS: http://www.geometryit.com/igis/

iGIS represents a new era in mobile GIS solutions by enabling users to load, view, investigate, create and export their own spatial data over a background of Google Maps imagery.

It was relatively easy to work with and seemed stable - granted I didn't use it extensively but the ~10 hours I did use it it worked well.