Having cut my teeth on C.J Date's 2nd edition, I'm a bit skeptical about MongoDB. Maybe I'm just not in a problem space suitable for MongoDB. What rules of thumb can you offer about when one should use a tool like MongoDB vs a relational database like Firebird, MySQL, or PostgreSQL? When might a system move from one to the other? Regarding your book, how much of it would you say is applicable to someone working with an RDBMS vs MongoDB?
"Hell hath no limits, nor is circumscrib'd In one self-place; but where we are is hell, And where hell is, there must we ever be" --Christopher Marlowe, Doctor Faustus (v, 121-24)
I have designed a few apps where RDBMS coexist with NoSQL datastores. For me, it is not one vs the other, but it is more of how they can complement one another.
It is more important to understand the needs of the application and that the flexibility/scalability of NoSQL datastores over RDBMS's comes at a price.
The general rule of thumb that I tend to follow is - when the application data demands for a tight structure and data integrity, go with RDBMS solutions. If the data generated is big, fluid(changing at a fast pace); consider using NoSQL datastores.