I find it difficult to seriously address comments to this thread. People still referencing 10gen and talking about data points from 5-10 years ago (2009, 2012, 2013).
You do realize it's 2017 now, right? Many things have changed in MongoDB (and in the IT world in general) during the last 5 years. MongoDB is a mature, established, full-featured database - recognized by Gartner as a "challenger" to more established RDBMS vendors like Oracle and Microsoft.
MongoDB has many built in features that are extremely attractive for building modern applications. Built in HA & DR, horizontal scalability, multiple storage engines, rich indexing and query capabilities, enhanced BI features in aggregation framework, and many more...
You do realize it's 2017 now, right? Many things have changed in MongoDB (and in the IT world in general) during the last 5 years. MongoDB is a mature, established, full-featured database - recognized by Gartner as a "challenger" to more established RDBMS vendors like Oracle and Microsoft.