I'm a big fan of relational databases. The problem with them is the high upfront cost of developing a new product: maintaining your schemas, updating data types, getting bogged down in data mapping code and worrying about what the most efficient SQL implementation is going to be, etc.

I like the fact I can whip something up quickly with MongoDB, get everything working and then once my schemas and services operating thereupon have stabilised then I can start looking at swapping my implementations out for a SQL-based back-end if I think it's worthwhile.