For your particular case, MongoDB sounds like a good choice, but there are plenty of scenarios (probably most of them) where it wouldn't be the best choice.
MongoDB is more suited in scenarios that call for reading/writing a lot of data, without much emphasis on transaction safety (if some data occasionally gets lost in a server crash, it's not a big deal), expect to scale big, and don't really have a stable schema.
MongoDB is not suited for scenarios that require:
- Strong ACID guarantees: MongoDB allows for duplicate data to be stored, inconsistent reads, and even data loss. These things are fine in some applications, but not in most.
- Multi-Object Transactions: MongoDB does support ACID transactions, but only for a single object/document. This just won't cut it for more complex operations like bank transfers, making a reservation, etc.
- Traditional BI: there are a lot of BI tools out there that only play well with traditional SQL.
- SQL: MongoDB has a very specific query language, whereas SQL is very well known by a lot of people (might be an important aspect to consider), can do a lot of complex things (whereas with MongoDB you'd have trouble performing a simple join) and is transferable across a lot of implementations.
MongoDB is faster and will allow you to eke more performance out of the system by eliminating a lot of stuff that RDBMS enforce by default, like integrity checks (note that you can also tweak RDBMS for such purposes, anyway), but the truth is, in most scenarios, it's just not needed. Plus, the trade-off is reliability and flexibility (you'll have trouble if, later on, you decide you need to do more complex operations with existing data).
It all depends on the needs of the application you're building. Is it speed and availability, or safety, reliability and flexibility. You have to know where in your data (and in your data's connections) lies more value. If you don't know yet, it's probably best if you choose something that won't paint you into a corner in the future, and will allow you to add the features and perform the operations your application needs.