Funny you mention the Forerunner comparison, Byk. I thought the "upgrade" from the 800 to 810 is similar to the 405 Forerunner to 405CX Forerunner. I remember waiting for the next Forerunner (because the touch bezel was horrendous). There was only one difference between the 405 and 405CX and was not worth the extra cash.
Unfortunately, I feel this is the same with the Edge 800 and 810. I'll wait to see if the 810 predecessor is worth my money, otherwise if the 800 gets a price cut, I might go that route. With the response from this forum already, I can't imagine the 810 is going to sell that well. It seems if the 800 drops in price, Garmin could do pretty well.
Comparing the cost of maps for the Edge and Nuvi, it does seem a very step up-charge for the Edge maps. Considering what the 810 could be, I'm very disappointed. It might just be worth purchasing the 500 on price cut, and waiting for the 810 predecessor (providing Garmin takes it where it can go). I have read that Garmin might have held back on the 810/510 in order to catch other countries up with Garmin products. It's nice for the other countries, but isn't to advancing for those of us with an abundance of the current model.