The latest issue of Flute Talk had an Ad on page 37 for Valgon Rings. No no, this is not a strange Star Trek device, nor is it related to the Vogon’s, whom destroyed Earth in Douglas Adam’s Hitchhiker’s Guide to the Galaxy. These rings seem to be a serious thing. To quote the ad:
"The Valgon Rings control and stabilize the oscillating air column on the outside of your instrument, resulting in significant improvement in your instruments response."
The website has a mere two endorsements, one from a Doctor at the University of Florida (flute), and some saxophonist named Colleen Allen who, they say, plays with Molly Johnson (?).
Anyhow, the saxophone version seems to be two “rings”, one that goes around the bell, and one that goes around the neck. Not sure the physics of this. Could it be that these rings are converting a saxophone’s sound to be more direct, and hence a “better” sound?
I think if you wanted to do it right, someone should invent some sort of scoup that runs the length of the saxophone. Something that would take the reflected soundwaves from the pads, and focus them forward. But hey, the easiest solution is to put on a reed and/or a mouthpiece that has more bite to it 
Sounds like new-age “snake-oil” to me. I doubt if Valgon rings make any difference. If it did have any benefit, all the professionals would be falling over themselves to buy them.
For saxophone, probably. However, it supposedly does amazing things for flute. A lot of people use them.