I'll sometimes contribute when I'm travelling to more rural areas which are less likely to be well mapped. The experience in my country has been that cities are very well mapped on OpenStreetMaps with a lot of detail, often having more up to date information than Google Maps. Less populated areas usually don't have as much detail, but the basics, like roads and buildings are usually well mapped.
I've also noticed OpenStreetMaps is awesome for trails and smaller roads used by hikers, usually being much more useful than Google Maps.