depends on the actual app use. Simple in and out low traffic crud? It's probably not worth the extra effort. MySQL is still a great product that has and will continue to serve billions of web requests and just cause NoSQL is a new kid on the block doesn't mean that everything should use it.

That said if you are storing data that is really just documents, Mongo or another document db is likely a good choice. For example if you storing interrelated performance measurements (like in a factory setting ) Mongo would likely not be the best choice but say in a setting like an app for applying for a job or registering for events ( natural document data segments ) a document db would work well for modeling the domain I'd think.

It really depends on the nature of you application, but raw speaking... probably you'll regret using mongodb as your primary database at some point.