I had a Vais brake installed on my custom .338 Ultra Mag, I installed one on a .300 Ultra Mag (both are the origional style). Yes they work. Others might be more effective at recoil reduction, but as far as noise I've shot mine up to 4 shots without hearing protection while hunting, and didn't have any problems. One shot from my braked .300 Weatherby will cause temporary deafness.
I have a Vais on my 300 RUM and love it. Shot it twice while hunting and my ears didn't even ring. My neighbors 300 Weatherby has a KDF and we tested both side by side with one of us standing next to the other while the gun was fired and the KDF had more blast coming back then the VAIS.
Recoil reduction was close, maybe the KDF did alittle better but not by much.
I have used S-1's breaks and it is better than any break I have used. It reduced the amount of recoil and torque. This allowed me to make follow-up shots off of a hardened surface rapidly. The reduced torque did not kick the rifle off the ground like the other breaks did. It also saved my shoulder from being turned into hamburger at the end of a long range day.
The KDF brake is not really all that effecient. Three brakes that some to mind are far better in recoil reduction and noise.
The Answer brake is the best so far for recoil reduction and is not as nasty loud as a KDF.
OPSINC brakes are very good for recoil and vent gas to the sides. They work better than the KDF for recoil.
Vais (Bartlett) brakes are much less offensive and also are better for recoil reduction than the KDF.
I have installed litterally hundreds of brakes over the years. I have cut more KDF brakes off than any other to be replaced by a better brake.
Other pluses about the Answer and Vais brakes are that they can be tuerned down and blended in with a barrel to make it look seamless. The will look like part of the barrel and not some big ole pickle on the end of your rifle. The OPS INC brake can be turned down as well if the barrel brake OD's are close.
I must comment on the two guys that said they didn't find shooting the brake without ear protection objectionable. I ended up shooting mine without hearing protection during an elk last winter. IT WAS A BIG MISTAKE!!! My ears rang for hours and I KNOW I have contributed to my continual tinnitus. I now shoot it without the brake when hunting, added a thread protector. If I KNEW I had time to stuff in plugs I'd consider hunting with the brake. Our sit and glass approach to coues wt could work with the brake.
I will say the brake is very effective and kind of sexy looking compared to that bulbous thing KDF makes.