Riak users have had quite the opposite results based on talks given at conferences and videos put online. Yammer, Voxer, Formspring, Bump are all riak customers and all have a video out there somewhere talking about how much the like riak. Yes riak and cassandra share a little bit of technology (being dynamo inspired), but most of cassandra woes seem to come from operational difficulties and not the technology theory. Riak has more of a focus on operational friendliness out of the box.

There are videos and talks out there from several customers listed on Basho's site, just take a look.