I've recently started carrying and using the Reelight 120 battery free lights and have had enough experience with them to offer a review.
I should preface all of the below by saying that my bar for lighting is very high. I've had the benefit of testing many, many lights and have had very good experiences while using powerful lights (no accidents, no near misses, no cars pulling out right in front of me). My track record with "adequate" lights is not as good.
The Reelight products are powered by magnets which attach to the spokes of the bicycle. The lights are attached to the axle or quick release skewer of the bicycle and are powered when the spoke mounted magnets pass by the light heads, which have a magnet of their own. The 120 series operates in flashing mode and stays flashing for up to two minutes after the bicycle is stopped.
These are not exceptionally bright lights. They are suitable for urban environments and other roads where traffic is not very fast. They are also good no-hassle backup lights.
Battery lights are a pain, and though they continually get more powerful, lighter, and smaller the battery that lasts forever won't soon be invented. For those of us who use a bicycle as a vehicle, the most logical solution is to have a lighting system that doesn't rely on batteries but is instead powered by the rider.
Generator powered lights outperform the Reelights by some margin, especially in the headlight department but are much more expensive. Even an inexpensive sidewall dynamo driven lighting system will cost more than twice as much as a Reelight set. An inexpensive dynohub system (front wheel with hub, headlight and tail-light will cost about five times as much as a Reelight set.
The lights aren't exceptional performers in the category of light output, but I'm very happy to have them for my city bike, which doesn't leave downtown except during the day and never hits big roads with fast traffic.
Impressive price, mediocre light output, and ready anytime you hop on the bike. Taken as whole I'd call the Reelight a good value as a backup system or primary lighting for bikes that don't mix it up in fast traffic.
Feedin' the family content: I sell Reelight 120's sets (front and rear light) for $55.