Pros: It's great for building your MVP. Super easy to get up and running, super easy to work with at a superficial level (I'm talking about storing data and querying here more than ops and administration), dataset can grow to a fairly large size without you having to think about your database at all, freeing you to think about your business. Mongo was my first brush with "schemaless" and that's a big advantage particularly for early-stage projects where things are in flux. I also enjoy using JS as the query language, because I like JS, but YMMV.
Also for certain use cases, particularly the single document case, it's probably one of the best solutions available. However, in my experience, this ends up being a limitation if your data doesn't fit nicely into a single document, and the types of data that do fit this use case are rare.
The cons you've probably heard before: no transactions, no joins, not ACID compliant, until very recently writes locked the entire database, scaling/sharding just works until it doesn't.