I also really rate my 820. It’s mostly rock solid. It’s switched itself off for no reason mid ride twice (only when stopped). I don’t know if it crashed or what, but it switched back on easily enough and hadn’t lost my ride data either time.
Battery life is great. I’ve only used it for about 7 hours (in one go) but it still had about 50% battery life left after 100 miles, with speed/cadence/hr sensors talking to it via BT/Ant+ (however they work) and connected to my phone via BT.
No issues with the navigation on it either – if you don’t go the right way because you have no sense of direction, it’ll tell you you’re off course soon enough and since you won’t be riding on any motorways (and probably not a dual carriageway unless you’re mental) then just stop, turn around and put yourself back on course.
New routes have never been a problem for me because they’re either singed in an event, or I took time to have a rough idea where I’d be going before I set off. Never tried the “get me home, I’m bored of riding” features of the navigation side of things, though.
My main complaint about it is if it’s got water on the screen (e.g. if it’s raining) then it can (rarely) decide I want to scroll to another page, as the drops roll around on the screen, or just refuse to scroll when I want it to.
I don’t like to ride in the rain, so … not a massive issue, but probably the main thing that annoys me.
In short, it’s great. And I’d buy another without hesitation.
Prior to this I had a 510.