I'm just a db4o enthusiast devlopper, nobody's perfect!
db4o is nearly complete and interesting in Embedded mode, above all with Java approach, with SODA query and Transparent Activation/Transparent Persistance.
Data arround 16 GB (after response time can decrease)
So, if you envisage to go bigger and bigger, you will replicate and merge different db4o containers in Versant database.
So you can have a look at Jira : http://tracker.db4o.com/secure/Dashboard.jspa to know projects/issues submitted.