OSM maps work with Garmin Zumos 590, 595 and XT. Possibly with earlier ones as well, and they also load and display Ok in Basecamp. Routes created in Basecamp transfer and load into the XT exactly as plotted in Basecamp- providing they do not recalculate.
In the Uk, there are very few trails and tracks that are navigable on Garmin maps. OSM maps have many more of these available, depending on the mode of transport selected.
There is a very big issue if you have OSM maps and Garmin maps for the same or overlapping areas selected at the same time in myMaps. The Garmin displays some very weird behaviour.
I've have a case ongoing with technical support regarding the XT wanting to take me back to the route that it is building behind me if. i deviate from a plotted route. It repeatedly asks me to U turn. When it does this, the recorded track log is broken.
They asked me to remove my OSM maps completely (not just untick them), and remove the SD card. Last week, I spent a few hours testing all sorts of situations in a location where this behaviour has happened every time until now. During these tests, the XT never once tried to get me to go back - it found a new route to the next route point - the one that I was taking. I reinserted the memory card and repeated the tests. It still worked.
The suspicion therefore lies with OSM maps being present, even though they were not ticked. To be be sure that it isn't some other change, I have to go back and retest with OSM maps installed, but not selected. But they have closed the road for 6 weeks for bridge repairs.
OSM maps for UK have no road speed information, so there are no speed limit warnings or displays, and the ETA calculations and displays are not reliable as a result. They are wring by a very long way.
But the maps have a lot of additional detail which looks good on the XT screen.