Some quotes:
"Unless you're already running tens of thousands of dollars a month in servers and think MongoDB was a better fit for the nature of your data, you probably wasted a lot of time for negative benefit. "
"MongoDB is fun to toy with, but it's almost never a better choice than Postgres/MySQL/SQL Server/etc. for production applications."
"It's beta quality and the list of deployments(
http://www.mongodb.org/display/DOCS/Production+Deployments ) is misleading. Most of those companies listed have used MongoDB for one side project, they aren't running their main services on it. Read the comments next to each company name -- one used it to track a single web form, another used it for an internal reporting application, etc. "
"One of the few that actually uses MongoDB for their main service is Foursquare, and that came back to bite them with days of downtime which would not have occurred if they were using a RDBMS."