This is the first GPS bike computer I have owned. I always got by with a speed and cadence pickup to the display on my handlebars. This is so much nicer with so much functionality. It was a "refurbished" unit, but it looks brand new and I haven't had any problems with it. Not looking back....
I tried it twice but I didn't like the way to change the pages. I had to do screen swipes which didn't work well specially when wearing a glove. I prefer in this regard the 520 and the 530. I didn't see that the 820 offered any advantage over the 520. About the same size. It worked well when I didn't try to flip pages or change function.
Pros: Mapping is nice, can configure the pages to your desires. Can setup multiple Bikes. Syncs easily. Has features to show text messages and phone calls. I got a NHO version so the price was fantastic on sale
Cons: The batter life is not nearly as good as my 6 year garmin 800. I'm slow but go long - this is gonna be an issue for me in the summer months. I also wish the device was a little big larger.
Use 820 for road. Great screen view, easy to setup, easy to use.