Overall I wanted to add two points
1) battery life: I purchased an aftermarket battery on Amazon for my android incredible. $50, adds a few ounces and a little bit of bulk, but effectively triples battery life. I think it'd be realistic to use my phone for at least 3 or 4 straight days as a navigational device without recharge in airplane mode.
2) airplane mode: on many Android phones you can completely turn off the cellular network separate from the GPS. This saves tremendous battery juice.
3) BC Navigator App (Backcountry Navigator, $10)… there are several GPS applications for Android phones that cache user specified topo GPS map data for free including USGS topo info, myTopo information, and USGS aerial photography among others. It's completely realistic to cache a huge area like Smokie Mountains National Park on your phone. This means your phone functions for all intensive purposes just like a Garmin and you need no cellular access, just GPS access.
4) Additionally BC Navigator can use another free and open source desktop application called Mobile Atlas Creator to create map packs including map data, waypoints, tracks and more and upload them to your phone.
5) MyTracks & tracking routes. Tracking on any device is a huge battery killer. However the excellent google app "MyTracks" on Android can be easily tweaked to conserve batteries. When I first got my HTC Incredible and used MyTracks in default mode it killed my fully charged phone battery in 4.5 hours. After I replaced my battery with a third party one (see point #1 above) I got 10 hours. Then after making a few settings tweaks in google's MyTacks application I triple that still to 30 straight hours! That's a minumum of three days tracking at 10 hours a day! What's more I haven't even tried it in "airplane mode" yet, which is to say I haven't tried it since I turned off cellular network access (see point #2 above). I suspect this will increas longevity to at least 35 hours and maybe over 40 hours.
6) Using your Android as you would a GPS or map… actually using your GPS as a navigational aid as you would a garmin or a map does not use up much battery life. Especially if you have it in airplane mode. If your not using it for tracking which keeps the GPS always on then the sleep mode is very effective.
I think that just about covers it.
I've got a big trip coming up in a few days and am trying to really perfect my GPS skills with Android. Am planning on using it for tracking and as my primary navigational aid, though I will be carrying a paper map backup.