Yes Brian, most of those apps - such as mapsme, osmand, mapfactor and a heap of others, some free and some costing a couple of dollars - use Open Street Maps and they are very widely used in place of dedicated car navigators by thousands of overlanders and travellers all over the world. Most of them are fully functional off line as well. In addition there are a heap of POI files that can be installed on both the mapping apps and on Garmin to make them even more useful. So much is free that there seems little need to pay for it, although for instance, OSM does appreciate donations.
The OSM maps can also be installed on Garmin Navigators and I have them for the whole of the Americas, most of greater Europe and Australia and as you said, in many cases they leave Garmin and TomTom maps for dead. One thing they don't always provide though is traffic, speed cameras and lane guidance although some apps have them available as extras.
One other feature of OSM and smart phone apps is you can log your tracks and send them in to OSM for inclusion on their map.