Garmin Edge 500 is a light weight GPS initially designed for bike riders. But it is also good for walkers. I put it in the small pocket at the top my sack that I carry everyday. For its performance test, I carried it on the way to back to home by walk. Below screenshots show its performance and accuracy very well in comparions with iPhone 3GS.
The first image is the GPS record of Garmin on my commuting route from my office at POVO to my home at TRENTO downtown. It shows my route precisely. On average, the route error was less than 10 meters. The biggest one is about 40 meters away from the route and I was walking under the tree. This route is like getting down from the moutain top. So it was like walking through the woods.
This image is the result of iPhone 3GS. I turned it on at the same time with Garmin. It only records a portion of my route when I am completely exposed to the sun light (where no tree over my head). No needs for comparison. Garmin win!