In reply to TobyA:
I've not used the jul2, but I did have a jul and it didn't always lock. On one occasion using half ropes (within the diameter range the device is designed for) the my partner took a fall and it didn't lock. I suspect this may have been because only one rope was weighted, but that is a bit of a guess. And when it didn't lock, it had very little friction and so made holding the rope very tough! Not too bad on that occasion as my partner was lighter than me and I could hold the fall, but would not have been fun if they were bigger.
I also found it an absolute pain to abseil on, particularly if you using a static rope.
Hopefully the jul2 has improved on these points. But for me the jul was a "jack of all trades, master of none" type device.