I've used these for a few matches now (IDPA and bowling pins) and here are my thoughts...
I tried these with .357, .38 Special and Short Colt (all 8-shot), .40S&W and 10mm (6-shot). Retention was good on all of them. One thing I like is that the little hump that's used for retention fits between two rounds so it keeps the moon indexed in the exact same position, meaning your fingers grasp the same spots every single time. The HoldAll also moves the moons further away from your belt compared to other options.
Retention also might be a little too good. The belt loop is pretty big and fairly flexible (it's folded-over kydex) so the whole unit has a tendency to bend outward if the moon hangs up at all during extraction. This sounds minor, but it effectively stops you from pulling the moon off unless you rip it out. A really sturdy belt helps prevent this from happening. The other minor quibble I have is that the diameter of the center posts are slightly larger than the diameter of the screws that hold them in. This creates a ledge that 6-shot moons can get hung up on. Not a big deal for normal usage but they would be a nightmare for unloaded starts.
Overall, these have more positives than negatives. I'm going to hold onto mine and use them for bowling pins (where there are no unloaded starts and I rarely reload). I'm sticking with my RSC posts for ICORE/USPSA and my modified Safariland holders for IDPA as they both have "good enough" retention but less of the HoldAll's negatives.