This was actually pleasantly surprising to read...the government actually wants to see if its software is useful and competently designed, rather than just buying it because the contractor has influence and no one will know how crappy it is anyway since it'll be classified.
I've worked with both HBase and Cloudbase/Accumulo. HBase tooling is more mature than Accumulo's but performance characteristics are about the same. The extension model for Accumulo - for custom iterators and aggregators - is pretty easy to get the hang of right away. (In fairness I never looked into extensibility for HBase.)
Both systems play well with Hadoop. Accumulo is a little bit more difficult to use with Oozie.
Accumulo's built in monitoring is decent.
But seriously, usage concerns for the project notwithstanding, it looks like a cool K/V storage system. I wonder how it performs compared to similar systems and how it handles failure.
Failure I cannot speak to since I really only develop against a local cluster that I blow away often.