I started using jsonschema at work and can't go back. We have an extremely large configuration file to configure our system and there are lots of 'optional' blocks that can be configured. Jsonschema really cleanly allows you to express that logic without having to write a line of code. It's been great.

Protip: if you are using jsonschema in python, it takes in a dict() not a file, so you can use yaml (or something else) as your configuration file and still validate it with jsonschema.