I have the cheaper garmin bundle and I've downloaded a set of openstreetmap maps. I use ride with gps for planning routes and download to my garmin as a tcx.
It all works brilliantly apart from ...
if for example, you are unsure of where you are and select, return to start , then it will work a route out to get you back, BUT it only uses the base maps to work it out, so possibly not the quickest.
The tcx gives turn by turn directions but I think the style and quality of direction does depend on whether or not it's base map or OSM/