Some of the detail is better and more current that the Garmin mapping, in other areas the Garmin data is better. Built in POI are pretty expansive and well. It lacks some of the extended data such as speed limits and can provide different routing as presumably the road characteristics are different.
Navfree that I run on my Android phone uses the same data and that works pretty well. There have been a few minor anomalies such as not turning right but requesting that going down the road, pulling a U turn and the turning left is the thing to do. You can usually see this as incorrect on screen and act accordingly. That only happened once though, pulling into an industrial area.
Taking my Nuvi 765 to Florida in October with OpenStreetMap data loaded.