Two weeks ago, I obtained a JBB and turned 45. Two weeks later i am now level 50, averaging 1 yellow per hour, without external buffs. The Jbb is the best thing for XP I have seen!
I have been soloing in DL and my tactic now is simply to slow the mobs, let them hit me whilst the pet melees and I cast the JBB over and over- as long as your AC is decent, your defense has been worked on and you have some sort of regen- you always win with the downtime being only that required to cann up the cost of the slow and any other buffs that wear off (make sure youre fully buffed).
It is great if you are in a small group but probably doesnt make much of a difference if you are in a full group as your role will be quite well defined (buffing team and slowing mobs) and if youre casting chain nukes by the JBB, you wont be able to med up for mana. Unless you have a KEI........
But well worth it for rapid XP gain. Probably wont make a big difference later on in groups but you could always sell it at the same price- I dont think the price drops all that much for one.