At sparkgeo we build next generation web mapping applications. That means we need to consider the mapping application from conception to delivery, from database to user expeirnce. Because we usually get to choose which mapping fabric we leverage, we will usually choose Google Maps.
We use Google Maps for almost any public facing web mapping product. There are billions of reasons (users) to support this choice, and the central driver for us is familiarity. Our users get to use a product they already know how to use, which means that instead of having to figure out the newest set of web GIS tools or layer layout UI, it all just makes sense. They get to focus what is being presented, rather than what is presenting.
At sparkgeo we went through the certified developer scheme (which is now depricated) for both the Maps API and KML, and we are now listed on the Google Earth Outreach developers list. We have used the maps API in a bunch of products, including:
and of course sparkgeo.com
Its important to remember that often we need to augment Google Maps with other geospatial or GIS technology. We migth be using PostGIS to manage data, a node.js backend to build and serve map tiles, some extra HTML5 geolocation support, or any number of other open, closed or mixed source technologies to build the web experience. However, when it comes to web cartography, Google has done an excellent job of describing the earth to the web using populations.