We are currently in the process of planning a similar upgrade from the oh-so-evil LDBM at the moment, and I have recommended using BDB as a backend. I did look into HDB, and while it looks better than BDB on all fronts [1], I did not recommend it's usage simply on the basis that it's considered 'experimental' [2].
My question to the list is: What are you experiences with HDB? Is it still experimental? Has anyone used it for an extended period of time in a production environment? Is there a roadmap anywhere outlining when it's expected to be stable?