I can attest to their support being phenomenal. I found a critical bug last week and got a patch within hours of reporting it to their customer advocate. You don't see that kind of turnaround very often.
Riak Search is in fact great to work with.
Something comparable for Cassandra exists with Solandra (https://github.com/tjake/Lucandra/). A major difference is that you'll have to post your documents manually to the Solr Instance running with Solandra while Riaks post-receive hooks automatically do that for you.
Also: the setup is by far not as big of a pain with Riak
Riak doesn't involve Java. It sounds trite, but that was a huge plus for our team. It has commit hooks. Search. Builtin JS VMs for mapreduce. More straightforward administration. Most operations are automatic or on-the-fly. Even though I think Cassandra+Hadoop is a technically superior product for managing absolutely huge gobs of multidimensional data (especially with its support for range queries), Riak's ease of use is pretty sweet.
Plus the team is superhumanly helpful about addressing bugs and gotchas. Everything's on Github, and contributing is pretty easy.