I am a big fan of mongodb but this is bad advice. The answer should be 'when I need a document-based data store'. You determine what type of database you need (RDBMS, KV, Doc, Graph, Column-based etc.) based on the type of data you are storing and retrieving. You don't want to use mongo for an accounting database etc. That said, mongo is the most mature, stable and feature rich of the open source doc based db's.
Comments
When should I use MongoDB?
Always.
I am a big fan of mongodb but this is bad advice. The answer should be 'when I need a document-based data store'. You determine what type of database you need (RDBMS, KV, Doc, Graph, Column-based etc.) based on the type of data you are storing and retrieving. You don't want to use mongo for an accounting database etc. That said, mongo is the most mature, stable and feature rich of the open source doc based db's.
You should read the three paragraphs after that comment.