Spotted this interesting article on Jython. I do think Jython's a neat way of scripting Java code. The article has some comparisons with using XML and Jython and it does look simpler to use Jython alot of the time.
There's a comparison of Jelly and Jython for creating user interfaces - this was the one example of where I thought the XML was easier to read & process. Afterall with user interfaces you often want to restyle & reformat; so using XML for this seems to make more sense - I found the Jython harder to read. Minor point though.
Another port worth considering is that Jelly and Jython work well together (Jython can be embedded into Jelly or Jelly can invoke Jython or use Jython expressions) so the two can work hand in hand.