Carlos, Why Hibernate is Better Than OJB, For Now:

How and why did this come about? My guess is that Hibernate's goal is "Build the best OR-mapping tool in the planet". OJB's goal however is more like "Build the best ODMG/JDO tool in the planet".

From the Hibernate Q&A

But what really makes Hibernate so good? It wasn't designed by a committee. Hibernate grew out of experience in an actual software project. It doesn't try to be all things to all geeks and so succeeds in doing one thing well.

Hibernate was brought up to 1.0 in a very short space of time, by two programmers working with one of the most efficient motivations a programmer can have: frustration. If any of you have ever done any significant work with Entity EJBs in Websphere 3.5, you'll know exactly the frustration of which I speak.

They weren't trying to produce a general purpose tool, they were trying to solve a problem. In my opinion, that is where all the best tools come from, because the tool never becomes an end in itself, the tool is always the means to some other end, in this case the efficient management of O/R persistence.