We are currently using db4o with a commercial licence, but finds it very frustrating after they have been taken over by Versant. The development of db4o is going very slow and major bugfixes are not been backported to stable releases, but only put into bleeding edge branches.
Therefore we do the bugfix backporting ourself. This would not have be possible if db4o was close source.
We have been testing ObjectDB and found that it would fulfill our needs. However we do have concernes in relation to your close source policy. Do you have any plans to open source ObjectDB ?