I haven't had a lot of experience with OODBMS's so I can't comment from experience, but it seemed that they were mainly built to "simplify" db access for developers. Rather than having the developer have to understand and store data in multiple places (and retrieve it), it's stored in an object structure and more easily retrieved.

They also have different indexing structures. I too am not sold that they really work well, but I have seen some implementations (can't remember them now, think NASDAQ or Chicago  BOT did one, and the performance was far above their old system. I'm not sure this says their approach was better or maybe the previous approach was horrible, but it was interesting.

Good overview Frank!