How useful are these when mapping off road rides, are they followable like a sat nav?

Real world users could I have your comments reviews please?

Thanks in advance.

Speaking about the garmin edge 810 here so I guess it may have slightly improved, I found that using it like a sat nav is fine on road (not used it too much for that though).
Using it off road it is possible depending on what type of route file you have there seems to be quite a few different formats and some work better than others (If I have an option I will use a Gpx Track), however I have never really got on with it I normally just turn the route I am following onto visible so that it is always visible over the map, and a quick look down at each junction/fork and 95% of the time you know which way you should be going. The other times if you move slowly down the way you think within about 10 yards you know whether you have picked the right one or whether you should turn back.

They’re good. They do proper turn by turn navigation, even off road, although you can turn it off and just follow a breadcrumb trail if you’re so inclined.

It will partially depend on the detail on the maps in your area. I have got OS maps on mine too, but other than looking pretty I don’t find they add much over the OSM maps it comes with.