Not because I wanted to buy Android instead of a dedicated GPS, but because I've already got four Android phones in the house and didn't want to buy anything.
In my case, I'm going back-packing for 5 days; we have paper maps and compasses, but I want to bring my phone along and see how it does. I have a small, portable solar charger that I'm bringing as well. Here's what I've discovered:
First, I'm taking my Nexus One. I have to take the phone with me anyway; I just won't leave it in the car when we hit the trail. With the screen off most of the time, but with the GPS on and a tracking application running, I got about 7 hours of continuous running before the battery hit critical. All wireless was off; theoretically, the only things running during that time were the CPU and the GPS chip. I used the display for maybe 20 minutes during that whole time. I expect that, with some coddling, this amount of time would be serviceable -- and it'd certainly be a fair emergency device.
The Nexus One compass -- the magnetic one -- is way accurate! I walked around a bunch with a Suunto Global magnetic compass, and the Nexus kept up admirably!
I think I tried every free or demo GPS map program in the market, and the one I settled on was OruxMaps. RMaps and Maveric are interesting and have useful features, but OruxMaps turned out to be the easiest to build up maps of my destination with, and it provided all of the basic features that I wanted. It has a built-in map builder which takes a little fiddling to figure out, but is pretty easy to use once you do. I did this all over Wifi (which is going to be faster than cell data, anyway), so no cell plan is required (although a WAP and internet access still is).
The display is the biggest battery drain, obviously. With that on constantly, you're not going to get more than an hour of battery out of it. However, the Nexus is smaller than any GPS with a color screen that I've seen; attach enough external battery pack (through USB cable) to make it as big as your average Garmin, and I think the battery life would be comparable. As others have said, the quickest and easiest thing to do is just buy a dedicated GPS; you'll get less for your money, but if that's all he wants to use it for, I don't think it's worth the extra effort to set an Android device up as a dedicated GPS.