I worked with MongoDB quite a lot in context of Rails applications. While it has performance issues and can generally become pain because of lack of relations features, it also allows for really fast prototyping (and I believe that Mongoid is much nicer to work with than Active Record).

When you're developing MVPs, work with ever changing designs and features, ability to cut off this whole migration part comes around really handy. I would however recommend to anybody to keep migration plan for the moment the product stabilizes. If you don't, you end up in the world of pain.