Product review (5 stars): Just set it up right
I was worried by some of the negative reviews but with the rock-bottom price, and free shipping(!), I was willing to give it a try. (By the way, it's the exact same item offered by at least two other sites at higher prices.)

Because other reviewers had noted an initial smoking problem, I carefully washed the diffuser in soapy water, then wiped it dry and placed it on my gas range at the lowest setting (warm). No smoking whatsoever, so I assume the manufacturer had coated it with some kind of oil to prevent tarnishing, which was stripped away by the washing.

I am now happily using the diffuser to get an even lower heat on my range, and it works very well. By the way, it nicely covers the standard 9-inch-square grates on my range, and placing it slightly off center (while still covering the flame) works for my largest pan, a stockpot, so I think the slight "bump" where the handle goes in might be a problem only for a very large pan or perhaps for a large pan on an electric range, where some of the direct heat source would not be covered.