Here's a quote from a recent blog post from Dare Obasanjo.
SQL databases are like automatic
transmission and NoSQL databases are
like manual transmission. Once you
switch to NoSQL, you become
responsible for a lot of work that the
system takes care of automatically in
a relational database system. Similar
to what happens when you pick manual
over automatic transmission. Secondly,
NoSQL allows you to eke more
performance out of the system by
eliminating a lot of integrity checks
done by relational databases from the
database tier. Again, this is similar
to how you can get more performance
out of your car by driving a manual
transmission versus an automatic
transmission vehicle.
However the most notable similarity is
that just like most of us can’t really
take advantage of the benefits of a
manual transmission vehicle because
the majority of our driving is sitting
in traffic on the way to and from
work, there is a similar harsh reality
in that most sites aren’t at Google or
Facebook’s scale and thus have no need
for a Bigtable or Cassandra.
To which I can add only that switching from MySQL, where you have at least some experience, to CouchDB, where you have no experience, means you will have to deal with a whole new set of problems and learn different concepts and best practices. While by itself this is wonderful (I am playing at home with MongoDB and like it a lot), it will be a cost that you need to calculate when estimating the work for that project, and brings unknown risks while promising unknown benefits. It will be very hard to judge if you can do the project on time and with the quality you want/need to be successful, if it's based on a technology you don't know.
Now, if you have on the team an expert in the NoSQL field, then by all means take a good look at it. But without any expertise on the team, don't jump on NoSQL for a new commercial project.
Update: Just to throw some gasoline in the open fire you started, here are two interesting articles from people on the SQL camp. :-)
I Can't Wait for NoSQL to Die (original article is gone, here's a copy)
Fighting The NoSQL Mindset, Though This Isn't an anti-NoSQL Piece
Update: Well here is an interesting article about NoSQL
Making Sense of NoSQL
