I've read some good things about this unit.  There are some great youtube videos that demonstrate its uses.

The main benefit of the JDX over DT's onboard out to PA that I see (and this is speculation that I am pretty sure is spot in) is this:

I believe the XLR out is using all digital modeling/LVM output resources regardless of amp volume/tube utlisation through the speaker.

The JDX connects via speaker cable between the amp and the speaker hence you are getting a more accurate direct signal to PA to what you created.

I would be very interested to hear the thoughts of any users of the JDX as well as DT users who may have more insight into the XLR out built into the DT.

Any spectators can read about the JDX here: http://www.radialeng.com/r2011/jdx.php

I actually have one on the way as we speak.

Thanks - PDK