Go read reviews on Amazon.com and form your own opinion (hint: my opinion was that frequencies embedded in mylar holograms combined with a silicon rubber band cannot improve athletic performance). That said, the placebo effect can be a powerful thing. Any supposed strength/balance demonstration/tests they give are a sham and results easily explained.
Example: "the torsional rotation test" - rotate your torso with an arm extended and make a mental note of roughly how many degrees you got around from the front. Now, put on a powerbalance band (or pick up a magic stapler, or do nothing) and try it again and you'll go further. Turns out if you try stretching certain areas of the body, wait a second and then do it again, you can go further than the first time.
The other demonstrations they give are easily debunked (again back to stretching out an area before adding the powerbalance band and then trying it again...that or strength/balance tests where the 2nd time around when you are wearing the band, you know what to expect and therefore perform better).
I wish they did magically work, but they don't, and these types of products/salespersons have been around since the dawn of humankind.