There are cheaper lights around, but for the size, this is one of the brightest I've seen. Comes with batteries. Can be seen from the side, not just from behind. I keep mine in a stainless steel container now and carry extra batteries. Need eyeglass screwdriver to unscrew back to change batteries.
Cost is high relative to other lights out there.
601 Titanio suspension Attive saddle, Profile Fast Forward seatpost, NiteRider Classic Dual beam up front. Get 2 to be seen and carry a couple of AAA batteries with you. For those of you who don't want to worry about the light after you've attached it, might want to think about a different light. Very minimalist and plenty bright. The chasing mode is not as visible as the flash mode, but it's cute, and it's easier to look at if one is traveling in a group.