I would love to go back to relational, but I can't get over the linear scaling and HA ease of Cassandra and I'm willing to write my applications to work around the data model limitations. Is there any open source SQL database (i.e. "newsql") that can linearly scale with Cassandra's ease right now? I look forward to CockroachDB but it isn't done yet.
I'm currently experimenting with Phoenix, because it's a relatively thin layer above HBase, but it doesn't provide many features that you expect in relational databases like transactions. SELECT support is quite complete, it can do JOINs, subqueries, etc.
I have recently discovered Trafodion which looks like a much more complete relational database, but at the expense of no longer being just a HBase client.
http://phoenix.apache.org/ https://wiki.trafodion.org/wiki/index.php/Main_Page https://vimeo.com/128190098