I've used it and been happy. If you want something that's simple to use, know your schema partly but not entirely, and you don't mind losing a few writes, then mongo fits.

Relational databases place such emphasis on reliability. Mongo is generally reliable but only two or three nines, and it's simple.

For example, if you're collecting sub-cent line items for invoices, you might prefer to collect 99.x% of the line items simply over 100% at greater complication and expense, particularly if you can measure x.