My wife has a set of older Monkey Lights on one of her bikes, and they're pretty cool. One downside is that the weight of them with 3 AA batteries is pretty significant, but it's not a major issue in the grand scheme.

They're definitely not the only lights on her bike though, and frankly, they're just toys that make nighttime riding fun, but don't add much in the way of safety or visibility.

If this ^^ is true, you should buy a decent headlight and taillight before adding more light to your wheels.