I actually have heard of them - it was recommended by Sara Wilson on her bulletin board. I was considering one for Penny Lane when I first adopted her - she would just shake and shake over every little thing.

I was talking it over with my sister who used to work for the Dept of Social Services and she said she had read some current studies that discounted the use of them. I don't remember exactly what it was - but I am having dinner with her tonight, so I will ask her again and let you know.