Reading about 510/810 now. Sounds interesting on paper. However, I already have live tracking through a free Android app, I'm not sure what's the value of being able to display the weather forecast on the Edge, and wireless activity upload is not a huge plus if you have to plug the unit into USB to recharge it anyway.
I'll be thinking about it more, but, at current eBay prices, I'd probably take 800 over 510.