I bought an 820 when the USB port on my 800 started to play up. I now use the 800 as a backup, carried on long rides. It does still charge, but only if the connector is pushed halfway in. I have since read that you can get the port replaced.
The main difference that I find noticeable is with navigation. On the 800, as you approach a junction the whole map will display, making the route clear. On the 820, you only get some text and a little graphic indication of what you need to do. I found the former much more useful, but have adapted to the 820, albeit its not quite as good in that respect.