Do Ultrasonic Pest Repellers Work ?

Bell & Howell have some out that claim to repel ants, roaches, rats, mice, by just plugging them into an electrical outlet, also Good Life and Black & Decker puts them out. They 'say' not to use them if you have hamsters, gerbils, etc, but that they are safe for dogs & cats. I wish they'd make something like this for fleas, However, they are not all that inexpensive but if they work, they'd be worth their weight in gold, not to have an exterminator come every month for at least $50. We live in South Florida so here, it is a necessity to have pest control, and Florida is just one big ant hill as it is. If you have used these, please tell me.