We went with HBase. Cassandra would have been suitable too, but we already use Hadoop for data processing so it was a natural choice within the infrastructure ecosystem. We will write a followup about that.
Did you guys roll your own HBase environment or did you go with the CDH? If you're using the CDH version and have any questions, feel free to shoot an email to cdh-user.
Cloudera has in fact been an inspiration for us to follow, you guys have really struck the right balance between open source and commercial support. We follow the same philosophy with Scrapy (an open source web crawling framework), as you do with Hadoop and its ecosystem.