Here's the scoop on the Shake Weight by the American Council on Exercise.  This is copied and pasted verbatim from their website.  ACE is the largest nonprofit fitness certification, education and training organization in the world with nearly 50,000 certified professionals who hold more than 55,000 ACE certifications.

Shake Weight

Manufacturer: Shake Weight

Price Range: $15.95 - $19.95

The Shake Weight^®, an “As Seen on TV” product, is marketed as providing “incredible results in just 6 minutes a day,” with just one piece of equipment.