An excellent real-life story of actual experience with MongoDB: http://www.sarahmei.com/blog/2013/11/11/why-you-should-never-use-mongodb/
I recommend that anyone considering a “document-based” database read this and thoroughly understand what Sarah is talking about…
…because the world IS relational.
This rebuttal was brought to my attention: http://ayende.com/blog/164483/re-why-you-should-never-use-mongodb
Personally, I find this statement: “And yes, this is a somewhat relational model.” amusing. Well, since it’s relational, doesn’t a relational database seem like the right tool? And what does “somewhat relational” mean? It either is or it isn’t, in my opinion. There’s no “somewhat” here.