I'm also using an 810 with OSM that I think I got from the website indicated by the original poster.
The main purpose for it was to pre-plan courses in order to obtain turn by turn instructions, and I have used it to log data on club rides, when following a leader.
In short, I found it very flaky, sometimes switching off, sometimes freezing (requiring a switch-off). When using turn by turn instructions it would sometimes want to take me in to a field, and sometimes fail to announce required turns.
I've lived with it for over a year now and find it "90% good". What I mean by that I haven't had a freeze-up / switch off for a long time, but it still has erroneous turn annunciation issues on most multi hour rides. What I can say though is the route that is graphically displayed on the screen has never been wrong, so I am now in the habit of keeping an eye on where I am expecting to go, not just blindly respondingto turn instructions.
Things I did:
1) I altered the settings to those suggested by
http://www.scarletfire.co.uk/foolproof-course-navigation-on-the-garmin-edge-800/ I realise the menu structure for the 810 is slightly different, but the same principles apply.
2) I always create my own navigation files in ridewithgps, I usually use .tcx files. Even if I plan a route in
https://www.cyclestreets.net/journey/ I reinput into ridewithgps. I'm not saying there is anything wrong with cyclestreets files, I just wanted a one standard process for producing navigation files. (Well not really always, I've recently had the courage to use .gpx files from cycle.travel with success).
3) I avoid losing the gps signal by either hiding the garmin in my saddlebag when I go indoors at stops, or switch it off then switch it back on again when I'm ready to go. This does result in the recorded information being in several files, but there are online tools to glue them together in to one file.
4) As mentioned earlier, I keep an eye on the display, not just wait for the prompts.
Summarising, I've found navigating with the 810 to be acceptable after adopting the points listed above. I do sometimes go off route and it seems to cope - not throwing a wobbler, and picks the route up when I get back on course. But I wouldn't go anywhere I didn't know without an alternate mapping capability, be that paper maps or a smartphone with a mapping capability.
And a final word about recovering from being off course if you want turn by turn help from the Garmin to get back on course. I recently read an article / post on the internet that I can no longer find, but it stated that the temptation to use recalculating should be resisted. Instead, manually create a waymark (pushpin) on the onscreen map and do some button pressing to get the Garmin to take you to the waymark, at which point it will resume the course that is in the navigation file, but I don't know which buttons.............
Hope this helps
Graham
