I have a 500, but have not used it since getting the 800. I should really sell it. What I like about the 800, (above the 500): larger screen, more screens, true routing functionality, as many bikes as you have, (I'm sure there is a limit, but I have six in there right now, I think), and button placement/touch screen capability. About the only thing I don't really like is that the screen material seems to be softer than on the 500. Somehow, shortly after getting the 800, I got a smudge scratch on the screen. The 500 has held up way better.

Another hearty recommendation for OSM maps. Free, already configured for Garmin, worldwide coverage available and getting better all the time.

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edit: Go to OpenStreetMap for worldwide coverage.