Has anyone used the stop light gadget for a motorcycle?

Yeah, people who use them get caught on a regular basis and it is a jailable offense.

all it is is a rare earth magnet that breaks the magnetic field at the lights

Don't waste the money - they don't work. The most common type of traffic sensor is a loop of wire embedded in the pavement that has an electrical current running through it. It will induce a current in any conductor that comes near it (basically it's a big metal detector). It DOES NOT DETECT magnetic fields or have anything to do with magnets. It is intended to detect the induced current in any large conductor (piece of metal).

There are a number of ways to increase the response of the sensor. The larger the piece of metal the better the response. The closer to the wire, the better the response. If the conductor is in a loop, perpendicular to the wire, the response is also increased.

The easiest way for a bike to trigger these types of sensors is to position your front wheel directly over the wire (you can see the cut mark in the pavement). This places a conductive loop (the rim) at the best place to induce a strong current. The engine (which is the heaviest piece of metal) should be as close as possible to the wire, too. Since most of the sensors are 6-8 feet across, most bikes can do both.

The makers of these magnetic triggers are ripping people off. Their gizmos don't work (any better than random chance), they know they don't work, and they are counting on the average person's ignorance of science to sell them.

I wouldn't spend money on one. If the light doesn't change and there's no cagers to trip it for you, just make sure there is a good break in the cross traffic and just go through the light.

Before they switched them over to motion sensors, they had the type that senses metal to activate the light. When I was in either the right of left side of the lane they wouldn't trip. If I backed up a bit and then pulled into the center of the lane, they'd switch. The middle of the lane at a stop light is the greasiest, oiliest place you can find, but better that than waiting hours for the next car to come by.