I just recently published an article describing my experience migrating from SQL Server to MongoDB, you can read it here: http://news.ycombinator.com/item?id=3203601

I agree with the author that MongoDB is green, maybe not quite ready for prime time yet. All things considered, I realize we took a risk by switching and while I am quite happy with MongoDB, I do worry that at some point we will experience a failure condition we might not be able to recover from.