[pessimizer]

When speaking about Drupal specifically, it means improving the rate of requests served 5x in some cases.

http://cyrve.com/mongodb

edit: Data stored in Drupal is way normalized, and read from a hell of a lot more than it's written to. I think in this case, when they weren't using it for general field storage, but just petitions and signatures which consist of endless names and links to user pages, the data was naturally denormalized enough. Nothing in that usecase would take more than three, maybe four joins. All it's doing in that case is just making upgrades difficult.