I used an early version of them. Had one I got from GG a couple of years ago, I think. It works well, better than the aluminum 'pole jack' most people generally use as there isn't a worry about breaking anything near the tip of the pole and, since it's a much tighter connection, there isn't the sway associated with the pole jack.
The way you use it: completely separate your trekking pole sections from each other (top/bottom), put the slender end of the extender into your top section, slide your bottom section into the extender, adjust to desired height and tighten bottom section/extender combination.
The only issue I ever had - don't know if they've fixed it with these - was that the extender doesn't fit all that tightly into the top pole section, so until you have it adjusted and in place (so that the tension of the shelter keeps it together easily) it can separate. Not a huge issue, you learn quickly how to deal with it, just a pain in the butt at times.
But all in all, well worth it. I've been waiting for them to finally offer these (I don't think they ever did, just a few people got a 'beta' version to play with) so I could buy a second one!