Most people who are working in object-oriented environments and ever had the chance to use object-oriented database management systems (ODBMS) love the easiness of how to acess those databases. OK, you've got to follow some constraints. Like the concentration on access per navigation and not queries. But you don't have to deal with the object-relational impedance mismatch to press your flexible objects into the strict corset of a relational schema.
The Tideland Smalltalk Object Store is a small embedable ODBMS offering different strategies for the persistency backend. The interface to the develeloper is almost like the ODMG binding.