I went for the 810. Deliberated over 510 or 810 but as the form factor is pretty much the same I went 810. Use the map function a lot more than I thought I would and knock up road routes to train on using bikehike.com. I’m a bit useless at remembering routes so it’s great for me as I get more variety of routes, and I hate stopping to check maps so not having to do that is brilliant. Very pleased with it and glad I got the 810 over the 510.
Have an old 500 too that I thought I’d still use for things like TTs, but with the bigger display on the 810 I find myself using that now and very rarely using the 500.
Edit: As for 810 features compared to the 800. I don’t really use the connected stuff, should explore a bit more. The touch screen is nice though and works surprisingly well.