I run two sites, one is the perfect use-case for MongoDB - http://www.AUsedCar.com , it's a used car search engine. We've seen nothing but benefits by switching to it from MS SQL Server. Queries are way faster etc... It's a great use case because 99.9% of DB interactions are read-only searches.

My other site, http://www.BudgetSimple.com on the other hand is using SQL Server (in the process of porting to MySQL). It would not be a great use-case for Mongo, because there are usually as many update, delete, inserts as there are reads, and instant database integrity and a schema are important.

Anyone that claims a tool is perfect for every problem is probably wrong. You need to figure out the best one for your use case, and load test, security test, performance test, etc... until you have a good guess for the right answer.