I've started doing this recently after selling my Zumo 660. I have an HTC Inspire using Nav Free that allows free download of great quality maps that seem to be every bit as good as was on the 660. (The maps even have bridal trails in the local state forest that are perfectly GS-sized. ;)
Yesterday I realized also that the latest version of Google Maps for the Android allows map download for offline use, which is a huge help. ALSO- since I signed in with my google account, all of the maps, waypoints, and routes I'd saved in Google Maps on my computer instantly came up on my device. (this is something that my brand new Zumo wouldn't do even after hours of trying.)
It's not a perfect solution, as screen brightness can be an issue as well as access to the power button when mounted in my RAM aquabox, but these cheap Android phones are miles ahead as far as software/capability. How well I can make the hardware work is TBD, but things look promising.