I've been playing with a DeLorme Earthmate GPS PN-20 for the past few weeks, and have been pretty impressed with it overall. I've updated my PN-20 review, adding lots of screenshots and observations, so I wanted to point that out. But one thing just blew my mind yesterday.
The image on the left below shows the PN-20 on a very faint grass path. You can actually see the trail on the aerial photo on the GPS screen. I had just arrived there when I took the screen capture below on the right. The trail is just to the left of the position indicator. It's faint enough that I might not have even seen it had it not been for the GPS.
Wow. That's really impressive. Now the trail could have been bigger when the one meter/pixel aerial photo was shot. In general though, it's not that unusual to be able to pick out things like desert tracks or anything that changes the light reflectivity of the earth's surface. The PN-20 isn't the be all and end all, but the aerial photos are a very cool feature. For the rest of the story, read my full DeLorme Earthmate GPS PN-20 review.