My best friend got one for her son, and she just absolutely raves about them. He stopped being so drooly, and therefore stopped having gross diapers, wasn't nearly as cranky...etc.
Now, mind you, it might not work for everyone. And if its $40, and it does, in fact, work, I would say its definitely worth the investment. And my friend got it for cheaper than that, but I think she got it on sale.