Jesse Wolgamott answers the question why should Rubyists be interested in NoSQL?
Once you reach the point in transaction system where the database is the scalability cause of your scalability problems, there’s no going back. You’ve taken the red pill. Table-based transaction databases are constrained by memory and there’s a hard maximum until your app crawls to a halt. The dream of true replication and easy sharding is built in.
Also: migrations just suck, even in Rails.
Interesting to note that Jesse’s talk is about MongoDB, CouchDB, RavenDB and Amazon SDB, the first 3 of them not being known for built-in scalability features. While that’s not to say they cannot scale — see for example scaling CouchDB — and while each of them has an attractive feature set, there are already other NoSQL databases that provide better and easier scalability: Cassandra, HBase, Riak, Project Voldemort.