While Postgres definitely has support for document-store like behaviors and can function in that capacity just fine, that's not the same thing as saying you can just substitute it for a document-oriented store like MongoDB.

Having said that, for someone who's already using Postgres, it's a great way to introduce some document-related behaviors (as opposed to strictly relational ones) by using the features it already has.

Even MariaDB is taking steps in this direction with many non-traditional relational types of storage options now available.