I've played with riak a bit (just in development, no deployment yet) and I have to say it's wonderful.
The Riak Fast Track tutorial (https://wiki.basho.com/display/RIAK/The+Riak+Fast+Track) is phenomenal. I wish more tools had this.
MapReduce interface is nice, links are great, status info is mighty, the distributed setup you get out of the box is fantastic, etc. etc.
Folks who need any kind of key/value store should definitely check it out.
