My wife came home with a box of four of these some months ago. They didn't fit on our sink faucets for some strange reason but I was able to get one to fit on our whirlpool tub. They are pretty cool for what they are - driven by some little coin cells and activated when water bridges two contacts in the body. The water certainly isn't as blue as in the picture unless you turn off all the lights in your bathroom. However, my almost-two-year-old loves the thing and always wants the water running while playing in the bath. Probably doesn't promote water conservation, but hey, it's a renewable resource, right? If I recall correctly, they come with two sets of coin cell batteries, I haven't had to change them out yet, but I think it has definately dimmed over time.
Oh, notice they show them affixed to faucets that come way up and out of the sink. If you have a relatively ordinary faucet that sits lower, think about the size of this thing before you buy. Even then, I couldn't get them to attach to my faucets, they were too big in diameter to fit up into where they need to mate with the threads. If your threads are more exposed, you'll probably have better luck.