I've been doing lots of research as well. I'm in the market for a GPS but trying to determine if I should buy a dedicated garmin (probably oregon 550) or try to get by with my Android. You can buy these apps for $10-$20 on the market. I could use the money I saved from not buying a dedicated gps unit and buy a solar charger. My thoughts are that I'll be updating my phone on a consistent basis anyway, why upgrade a gps unit all the time if I can just use my phone. Garmin better watch out, selling dedicated $600 gps systems will be a thing of the past eventually once these smartphone apps become more mature.
Looks like GAIA GPS and Backcountry Navigator are turning up as the most popular, at least for supporting android. Besides basic functionality of a GPS like tracking, compass, etc. I need the GPS to tell me if I'm on public or private land and be able to show me gmu. I just don't think these phone apps are there yet; but please speak up if you know of an app. Accuterra claims to show public/private but their website hasn't had any updates since early 2010 so I think the project may have been abandoned.
I just don't think a garmin with huntinggpsmaps can be touched by any of the smartphone apps; at least from what I've been reading.