I am using postgres and redis - they are both trivial to set up and cost about a few lattes a month for a fairly scalable solution. If I had huge traffic, a few hundred bucks a month would be totally adequate - and this is running on the too-expensive heroku. So, unless Riak costs pennies and takes literal minutes to set up, the 10x claim won't hold.

Riak looks like it would be overpowered by my needs - but then again, I learn something new every day.