In reply to TobyA:
My MegaJul was super-grabby on my fat, worn 10.2 Edlinger. My new grigri2 was so grabby on that rope though I bought a new 10mm Yugi (haven't tried the combination enough to test it).
On two 8.5mm halves, the MegaJul locks and holds my weight when abseiling. I have, however, observed: a) it does NOT hold my weight on a single 8.5mm half, and consequently I don't expect it to lock reliably, b) it also crept down the rope abseiling on someone else's single (diameter unknown).
I like it, and use it for trad (used to use it for single ropes) and for abseiling (on known ropes, usually without a prussik backup), but it has definite limitations and it is NOT a grigri.
In any event you need to trust your belayer, not their belay device... a good belay device won't save you from a bad belayer.