Can't comment much on the JDO part of Castor,
since I've been mainly following their XML thread.
But on that one I can tell that they've been *way*
ahead of JAXB for a long while -- while JAXB was
stuck with DTDs and not making any progress,
Castor was happily supporting XML schema and had
a working implementation. From what I see today,
the Castor XML marshalling framework still seems
easier to use than JAXB (but this may be just a
personal preference of mine).
Another example of where Castor is ahead today --
it exposes (and implements) an API for XML schema
manipulation (JAXB doesn't provide that).