I use them and I like them. I first discovered their benefit when I went to the range to try out my new Magnetospeed V1 chronograph. I was shooting my 6.5-06. At first, I shot the rifle just to make sure it was still on zero and shooting good groups. I then strapped the Magnetospeed onto the muzzle to test it out. I knew there was going to be a shift in my Point of Impact (POI), but wasn't sure how much. When I went to check the target, I found the POI was about 4" at 200 yds, but what intrigued me more was that I had just shot my best group ever with my 6.5-06.
I repeated the process several more times just to make sure it was not an accident, then I went out and bought de-resonators for each rifle I owned (they are only $10-12 each).
I have found that the smaller the diam. of your barrel - the more pronounced the benefit is. My 243 and 6.5-06 now shoot with smaller groups. The benefit to my 338 Lapua Imp. is inconclusive, but the the diameter of that bull barrel at the muzzle is still 0.92".
Just follow the directions. Start an inch or so back from the muzzle and shoot a group. Keep moving the de-resonator back an inch and shoot another group. You will find the sweet spot pretty quick. Once you have your sweet spot, you can then repeat by moving forward and back in smaller increments to fine tune. Good luck.