I have and like the 820. No issues with bugs and it suits my needs (training and navigation) better than the 520. Mrs O has a 520 and I previous had / still have in the drawer, a 510, so I have experience of both of those.
The only issue with the 820 is the price,
Wiggle have the head unit for £308 at the moment, which seems a lot. I got mine for £250 at the start of the year.
Oh and if you're wanting to get something that is compatible with Stages power meters, look elsewhere. Stages will blame Garmin and Garmin will blame stages, but they don't work well together. I sent back my Stages as it was rubbish. No issues with any other PM brand I have used. Lesson here was avoid Stages.