MongoDB is new and untested? What rock do you live under?
It is new compared to many of the veteran Relational databases like SQL Server (and I don't mean that in a bad way), but it is a proven technology used by many. See http://www.mongodb.com/who-uses-mongodb
Mongodb is a good db but it is not good for many use cases that people are using them for. There are a lot of companies that realized the mistake and are actively migrating off it. I know we are planning a "Off-Mongo party" with another company once we both manage to migrate off
Many of us who used MongoDB have concluded that there is no task at which it is in any way better than all other existing alternatives.
At my company most of our systems are SQL Server powered, but one of the newer systems that stores large blobs of metadata for products is using Mongo, and it is working quite well.
Many of the companies that switched off MongoDB were growing and ended up moving up to databases like Cassandra. MongoDB is a great database from when you're starting to when you're mid sized.
Cassandra destroys PostgreSQL in scalability but we don't say PostgreSQL is a crap database because of it.