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.
DynamoDB does not directly correlate to Cloud Bigtable, it would be more akin to Cloud Datastore (which I don't think has a minimum and has a free tier)
They're pretty explicitly saying that this isn't an entry-level database—which is perfectly OK. For example, in the docs it says that Cloud BigTable is not a good solution for less than 1TB of data. If you're storing more than a terabyte of data and need the benefits that BigTable provides, you're probably already spending at least $1,500/month on your current database.
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.