I have one, a "Power Band". Got it through a "promo" for free, and that is, IMHO, all that it's worth. (I'd received a $10.00 credit for signing up on a "daily deal" site, and the majority of items had terms that do not allow the use of the credit for signup to purchase that item. When they had this, I used my free credit. (I have since unsubscribed from the site. I did not receive the two gift cards I purchased after 60 days, with multiple lies from their customer service, and finally chose to go through PayPal's "dispute process" to get my money back. That took about 1 week! )
Anyway, I tried it for a few days, and found it annoying and uncomfortable on my wrist. I definitely would not reccomend it, and would discourage others from purchasing one, regardless of price. Yet, there is one "old man" I occasionally play poker with who wears one on each wrist, and swears by them.

The "FAQ's" (from their self serving website PR) on the Power Band: