I used RDMSs for 15 years before ever using Mongo. I fell in love with Mongo from the first time I used it with a pre-existing system. The beauty of just being able to insert and retrieve my whole C# object graph with one statement was a thing of beauty.

I'm a Domain Driven Design true believer. The thought of just being able to store my entire domain model without the machinations of the object relational impedance mismatches was glorious.

I use C# so my document collections are strongly typed C# objects and thanks to the Mongo C# Linq provider, my queries are also strongly typed.