At one point in time the OODB's where making quite a bit of buzz but it quickly died down when most of the database vendors added features to allow similar storage options. We used one a long time ago and it leaked memory bad, it was a horrible experience keeping the thing together and running. I am sure they have improved since then, but with the features now available in existing databases, I really don't see the value in investing the effort to go over to a pure OODB.