It depends a lot where you go. In places like Nepal OSM is often better than any map you can buy. In the Europe and especially the US, it's not as detailed as the national maps. They usually also have much better contours (drawn from aerial photos not from the space shuttle).
But still much better than Google maps, which seems very much for cars (at least outside of cities).
But still much better than Google maps, which seems very much for cars (at least outside of cities).