Schemaless NoSQL database are a joy to use because they make it easy to iterate on your app, especially early on. And to be honest, the relational model isn’t always the best fit for real-world evolving and messy data. On the other hand, relational databases are proven, robust, and powerful. Also, over time as your data model stabilizes, the lack of well-defined schemas becomes painful.
Video Producer: http://railsconf2012.com/