Understanding HBase and BigTable
This is a very good article on HBase http://jimbojw.com/wiki/index.php?title=Understanding_Hbase_and_BigTable. As Jim says in his article, after reading it you should be better able to make an educated decision regarding when you might want to use HBase vs when you’d be better off with a “traditional” database.
This is also a nice primer on NoSQL in general (at least for system like Cassandra, etc).
What I really like best about this article is how Jim breaks out the terminology and working it to explain what HBase does properly.