The same example I provide everywhere. I have successfully used Mongo as a geospatial data store, where I have a public transit app/website, and most objects/documents have geospatial properties.
Sure, there's PostGIS, but saving a GeoJSON multiline which I can slap an index on works for me.
I have grown fond of Mongo because I've been using it for almost 5 years now. I know its limitations, and also what I can do in RDBMS that I can also do in it.
I sometimes build OLAP logical models at work for clients, and most of what I touch at work is SQL, but sometimes when I start a project I use Mongo instead of SQL.
It's only one use-case, but I hope it is something.