In reply to steveshaking:
> Its not easy to understand these comments from Jim Titt; I need pictures, graphs and tables.
Seriously? They are in the thread.
> So I don't really understand the problem or the point at which the holding force is lost. But he is drawing these conclusions - so it would be good to see his data and methods to make sense of all.
It's in the thread!
> In the real world is anyone finding these devices slicker than ATCs or even the grooved versions?
If you read the thread (and can navigate past the static), you'll understand there is more than one "real world." The "real world" most people speak of is the world of "everyday" falls, ones that are relatively easy to stop either because the fall factor is low or because there is a lot of friction in the system or both. Then there is another real world of "extreme" falls, ones with fall factor greater than one and with very little friction in the system. These "extreme" falls are rare enough (one condition is that the leader has to fall past the belay) that plenty of folks never have to catch even one, but if you do you will have entered the second "real world."
In the second "real world," a big question is when and how much the rope is going to slip. The surprising answer from Jim's data is that the rope will slip sooner and more with a MegaJul, Alpine Smart, or Alpine Up (listed in order of decreasing bad performance) than with a "non-locking" ATC XP. The fact that the first three devices "lock" at lower loads does not mean that they "lock" at higher loads, and moreover once the rope starts to run through them at high loads they provide less braking assistance than an ATC XP.
I don't think we know what "the problem" is (well, maybe Jim knows...) My hypothesis, based partially on comments from Jim---but these are my speculations, not his so don't shout at him---is that the "locking" mechanism of these devices involves pinching the rope in a hole between the carabiner and the body of the device, and at high loads the rope stretch makes it thinner, but the hole has a minimum size and so at some point no longer provides adequate pinching. At this point, the devices revert to ATC braking coming from friction and bends in the device. But in order to make the hole-pinching mechanism work, the carabiner has to travel much closer to the top of the device than in an ATC (all the devices have a deep slot the carabiner travels in towards the top of the device), and in that high position the rope bends that have to perform the braking function for high loads are gentler then an ATC's, leading to worse extreme-case behavior.
Anyway, that's my understanding of the data and charts from Jim in the MP thread. One conclusion I think worth considering: if you use one of these devices in a multipitch environment, it is may be more important than usual to wear belay gloves.