I have one of the Brake Free lights and it uses an accelerometer, it's NOT just a basic tilt switch. Nor does it rattle when riding, it's a very solid unit.

I've had friends following me (both in cars and motorcycles) who comment later they can't believe how much more visible it makes me.

It does attach to a helmet and require that you periodically recharge the battery but I'm very happy with the purchase (as opposed to just flashing the brake lights on the bike when slowing)