The biggest lure of Mongo is that it gives you a nice SQL-like query API. So it's fairly easy to get started with, compared to other NoSQL alternatives. I primarly use it for small-medium size apps - when I know upfront that I will never need to scale it beyond certain number of users in the short-medium term.

It's not as bad as it's made out to be. It's only if you really are looking to scale out, you should probably be better of picking something else.