Molten whistles are supposed to give off a higher decibel level at a much higher pitch, primarily to make it more audible in matches with tens of thousands of people in a stadium. I think I heard that a lot of FIFA Refs have started using them.
I know one guy who was given one as a trial but he didn't like the obscure shape/size and the change of pitch.
I've only used the Valkeen, so I can't judge by other Molten whistles. I quite like the shape and finger holds. I'd even go so far as to say that it's the most comfortable whistle to hold that I own. The hold sits between your fingers, with the body of the whistle on the palm-side of your hand. The body flips up for whistling, and you simply hold it between your thumb and forefinger to whistle (like any other whistle), and when you aren't using it, you can just let it fall, or flip it back into your palm while running, or even while writing. It's very comfortable, and intuitive.
As for the tone, I find it much less harsh than say, the Fox Sonic Blast, Fox Sharx, or even the venerable Fox 40. In fact, at a U19 tournament last year, I had a player actually tell me she liked the tone of my whistle. That's certainly never happened before. It's a dual-tone whistle, in that there are two holes that provide two different frequencies that create a very penetrating, loud, but pretty pleasant multi-tone.
But now I sound like a salesman...haha. If you get a chance to use one, I do recommend it and see if it suits your style. I really like mine, but you never know. I know some referees that will never give up their ancient tin whistle with cork ball...
