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That doesn't sound right. Every significant NoSQL deployment I've heard of was either a replacement for a relational DB or is a task that can be solved by a relational DB with some work. This includes Google's usage of BigTable, Facebook's usage of Cassandra, Digg's usage of Cassandra as well as reddit's.

Which deployments are you thinking of when you say NoSQL DBs are not competitive with relational ones?

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I believe the OP was referring to the fact that if you are replacing a RDBMS with a NoSQL solution then chances are the RDBMS was not really a good choice in the first place. If you need the CA section of the CAP triangle and can't pass a possible inconsistency to the application level then a RDBMS on a big honking box with master/slave replication is probably your best bet -- a lot of the NoSQL traction has been in providing solutions to people who don't specifically need that part of the equation but fell into a RDBMS because it was the default/traditional solution to "I have some data, now how do I access and store it?" question or because they did not know that options which relaxed consistency assurances were not quite the heresy their DBAs claimed.

That's strange that you suggest master/slave replication as a way to achieve consistency and availability. If you lose your master, you can't take writes and are thus no longer available. And if your slaves are behind the master, then reads from the slave are stale, definitely not consistent with the master.

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