I have the SL120 model, which has a feature that earlier models don't, which I'll mention as we're going along.
This is a really great product, especially at this time of year when nights are long, weather is poor, and visibility is limited. Any sort of light you can add to your bike is a bonus, and the Reelight is a perfect visibility enhancer.
It is NOT a headlight. They are simply two flashing lights, one white one for the front and one red one for the rear. As has been mentioned previously elsewhere, they require no batteries and nothing that will take energy away from your forward progress like some of the battery-less lights we've seen in the past. It uses elecrodynamic induction to create the power, and I'll let their actual site explain that better:
The lights are based on the electrodynamic induction principle, and work when you screw two magnets on the spokes and fix the light to the wheel’s hub. Electric current is then produced when the magnets pass the light, which incorporates a spool of copper wire. The light then flashes every time the wheel rotates. Simple, effective and ingenious.
And it is just that. This quote is a tad misleading, but in a good way. When I first got the light, I thought that it would blink twice with every rotation of the wheels, at the point the magnets that you attach to your spokes pass the light. That is NOT the case: the magnets simply power the internal dynamo, and the lights flash at a consistent speed - even AFTER you've stopped moving, long enough for most traffic light stops. This is that feature that the SL120 has that the earlier model does not - once you stop with the earlier model, so does the light.
Installation was pretty easy, though I would recommend that you get your bike onto a work stand to do it. Not having one, I did it with the bike on the ground, and that was harder. The directions tell you to have the magnets (which attach fairly easily to the spokes) 1-3 mm away from the light on the rotation, and having to adjust that a few times by picking the bike up slightly, turning the wheel, and checking the distance made it tough. Do this with a stand. And they're not kidding about the 1-3 mm. The closer the lights are to the magnets, the better, as the lights will power on sooner.
Operation is easy - just ride your bike. They take a few rotations of the wheels to get re-charged, but once they're on they're on for a while. One thing I did notice when I first got them: with a shorter commute like mine, it takes a while before the lights get powered up enough to stay on at traffic lights or stop signs as I mentioned above - a couple of days. A longer commute or ride will power them up faster. That is to say, that for a couple of days, the lights will actually turn off more quickly after a stop until the dynamo gets enough charge to store up enough power to keep flashing at a stop. I hope that's clear.
The front white light seems to do better with this than the red rear light. Your results may vary here: my red light is a tad further away from the magnets than the white light, so that may have something to do with it.
The mounting of the lights is excellent. One of my concerns with the lights was that the rear red light wouldn't be easily seen underneath my panniers, which are fairly large. That was not an issue - though I wish it was easier to check back and make sure that the light was flashing when my panniers were on the bike. I'm a careful guy, I just like to check on occasion.
I'm impressed enough with this product that I want to order two more pairs for my wife's and son's bikes as soon as things get a little more light out and they're more liable to ride. They are totally maintenance-free and I like the idea of having the lights available 24/7 for their rides. They are a bit pricey compared to other blinky lights, but to me the maintenance-free nature and the power of them makes up for that.
All in all, I HIGHLY recommend this product. I use it in tandem with regular headlight and a red blinky that attaches to my rear rack and I've never had a problem being seen. I bought mine at Pricepoint.com, but other outlets have them as well, obviously.
People, not speed.