NoSQL — application features

We have a huge distributed project. Users, roles, meta-data in mysql, everything else in couchdb.
Very good living.
CouchDB views allow you to do the app is incredibly flexible, without spending time thinking through complex internal relations, creation of composite indexes in the database, etc.
And due to the very handy tools for the administrator(an amazing web admin) and master-master replication literally 20 minutes connected to the additional server(100+ requests per second, several gigabytes of data, 2 servers).

IMHO damp even on sites with poor load supply is in principle possible, purely out of interest, dig deeper, imbued with concept.

"NoSQL systems" just what you want to learn.

As a user of CouchDB I will say that:
1) is Justified when you are in advance are unable to accurately determine the data structure, or the structure will change frequently;
2) Is, if your data fit well to bond wood shaped storage system.
3) I have not encountered that were involved in two SQL and NoSQL systems simultaneously. But that doesn't mean it's impossible. Just you will be hard to think in two different directions.

Use MongoDB. Much simpler and faster SQL.