I got two to try from a distributor in the states a couple years back one for the gen4 and one for the gen3 as I was considering opening up a business and these were one of the products I was going to sell. They come with longer pins that replace the back most pin in the receiver.
Overall I get the idea, the product is floppy/weak but it kind of has to be in order to get a good fit around the multiple glock frames each one claims to magically fit. As contrary to glocks generic information there is differences in over all widths etc between models, even of the same calibre.
My issue was that it kind of defeats the purpose of the slim frame when you fill the gap up with a fake beaver tail, my hands fit a size large glove, but I find most glock frames cumbersome, so on the g22 gen 3 it made it even harder to hit the mag release button. It was ok on the g22 gen 4, but took away all the advantage of the fact that I didn't use a backstrap on the gen 4. Like a couple of the other posters stated, I've never had a slide hit the web of my hand and I suspect this is only something that happens with bear paws and Andre the Giant Hands.
Granted the two I have are early release ones, they came in full packaging and were supposed to be of good enough quality to convince me to buy hundreds of them, so it should be noted that there is little plastic around where the wider extension pin fits thru the frame, after a box or so thru the gen 4 the strap holes stretched, not enough that it fell off, but enough that it didnt fit tight and could get cought on something. Also adding this to a production gun would kick you out of production class, so the venture seemed kind of weak even if they were dirt cheap. I even pondered putting them on the EE, but didnt want the hassle for the $10.
I guess if you've got $12 to spend; sure, go ahead and try them, but I think they're cheesie, the smart money is on a nice precut grip tape kit. Really, if the glock is biting your hand buy an M&P or 1911 and get over it.