> Can someone suggest when it is actually useful over a properly tuned relational database?

Maybe when developers don't want to bother to learn SQL?

On my CS degree we got proper introduction to SQL and the relational algebra behind it, so for me it is just a tool, not a scary monster.

I really love the data integrity options I have at my disposal with relational databases.

So for me this all NoSQL fad never made much sense. Then again, I never had to deal with a Facebook like scale problem, only daily reports from mobile network operators across their whole network elements.