I have both the 500 and as of last fall the 820. I have not been able to really use the 820 much tho since my motorcycle accident has resulted in only one outside ride since October as of today. I have used it on trainer a few times and the option to choose indoor is a nice addition.
The times I've used it outside (3x since purchase) I like the map feature. I'm still dialing in the screens and what I want to see on them. I had my 500 pretty dialed in as to what I liked. It is still functional and I like it for its simplicity as compared to the 820.
One feature of the 820 was the friend tracking...tho I have no clue how that works or how to set it up. It would be handy for my team group rides. Last summer training for Cool Breeze one of the riders got lost and was not understanding route ... would have been nice to be able to know where she was.