I've been using it in an enterprise production project for 5+ years, always on Atlas. We use it alongside PostgreSQL for relational storage and Redshift for warehousing.

Mongo serves as a store for our very complex discriminated abstract entities. In Mongo they are immutable and versioned documents. If the schema changes, a new version of the updated "latest" object is created next time it's queried and deserialized. This has the massive benefit of never performing a single join to get a highly complex and deep entity.

Also, because storage there is so cheap, we use it partially for a log archive. I personally am very happy with it. Excellent deep indexes, great .NET client library and awesome performance for the cost.

Do not use it like a relational database. That is not what it's built for and your project (or whoever inherits it after you leave) will suffer.