Product review (5 stars): Best to leave a bit of it out until you ...
My Crumb Stop has arrived! Delivered on time... It feels and looks just as expected, a medium hardness black rubbery material that is strong enough to solidly keep the barrier in place inside the gap, but soft enough to allow you to squeeze it into the gap without much effort. It took a second of head-scratching to figure how to do it, but eventually I got it: you start at one end by squeezing and inserting that end into the gap, then continue to squeeze/push in progressively toward the other end, making sure not to push it too deep. Best to leave a bit of it out until you inserted all the way to the other end, then gently push the rest of it in until level with the edge of your counter top. Make sure it's in an upright W position, so it can catch anything that might fall in there...
I love that it is black and it goes completely inside the gap, so you barely notice it. Additionally, you can wipe the counter and the stove tops without fear that stuff will fall inside (well, it may but it will be caught by the crumb stopper), and without any interference from the device, like it is the case with other ones I saw.

While I didn't need to do this, the rubber is soft enough that you can trim it using a pair of scissors, in case it is too long, or if you want to reduce its height (e.g. if you have a very thin counter top edge).

All in all, I like the product and would recommend to anyone who likes to keep things clean around the kitchen.

Update: we just made dinner (we like to cook most of our meals) and I can already see tiny bits of salt and a few drops of oil that jumped out of the pan that were caught by my Crumb Stop! Normally they would have fallen inside the gap and in time would have made a mess in there that would have had to wait weeks or months before the next clean! We have a stove which is very difficult to remove to allow cleaning of that hard to scrape caked mess that forms in there in time. I am thinking to buy another one to cover the vertical gap between the stove and the lower part of the counter, too.