If you test it at the LBS and it doesn't just about blind you when you're holding it in your hand, it's pretty worthless.
For tail lights? Planet Bike Super Flash, Cateye TL-LD1100, and maybe a few others. If the light isn't in the range of brightness of those, it won't be very visible unless you're all alone on a dark street. Otherwise it'll just get lost in any clutter or won't be visible at all.
Head lights need to be bright enough to be seen - unless you spend a real good bit of money they won't actually be bright enough to provide enough light for you to go much more than about 15 mph safely.