I bought 3 sets of the 14mm Bavaria models from Rids Bruners, and they have performed excellently, if a little deformed on one or two feather tops where I mishit in a tight spot. But they're very tough for their size, and all 3 sets and tether cords will fit in a 100mm snapseal box. They were particularly useful for enlarging a vertical squeeze in a mine that was just too tight for safety. But it did mean drilling and hitting pretty much head-first in the foot-wide vein cavity, so it was arduous. But I managed to shave off about 100mm with two 'sessions', using all three in a line. Also dropping them was an issue so I used tether cords with elastic bands, though they don't always work. I found a telescopic magnetic wand with a LED light on the end, and it proved very handy for retrieving a feather from amidst deads on the floor.
They're certainly no effort at all in use, and are more akin to ringing bells or playing the xylophone than smashing rocks. If they used plug and feathers on a penal colony, it would be a pretty easy time. There is a limit to what they can do though, and on certain attempts, it's just resulted in failure - the rock was either too tough (under compression) if bedrock, or just too big (if a boulder) - in that case you either need to scale up or use something else. Trouble is, I think his next size up is 18mm, and whilst fine above ground, drilling 18mm holes underground could prove draining, in every sense.