I played with both db4o and Perst for Java. I am having problems with the Perst method for processing object updates, so I already visited db4o; I'm still in the early stages.
I heard that Cache is a good OO database, but it is not free. It provides many features that RDBMS have that many open source OODBMS do not have. However, I did not use it because of the cost.
The biggest problem with OODBMS will be finding good documentation, good ACID capabilities and solving scaling and disaster recovery scenarios. Special requests and reporting are also complex, so this is definitely a plus to ensure that these types of requests or any reports are not needed.
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