I think using the HBase API is a very clever move. This means that the HBase API is now supported on AWS (EMR), GCE, VMWare (Serengeti), OpenStack (Sahara), and everywhere (Hadoop, if you're willing to run it yourself).

In comparing against DynamoDB (for example), you'll have to weigh a proprietary single-vendor API against an API with a good open-source implementation (that will get even better with hydrabase), yet that is also available in managed-form on all major clouds.

Edit: although - ouch - the $1500 per month entry price-point does not compare well to DynamoDB's $5 per month minimum.