I have one of these so called magnet things supper glued to my lowering link. Some lights change for me some don't, and seeing as I'm in a new city there is nothing to say that the lights that do change would have done so even if I didn't have the magnet. The sensors here are magnetic, and I was taught from the MSF to try to place as much of the bike on the open slits (covered by road tar) because this is where they bury the electronic wire.
Mine was free, but I would say for the hit and miss aspect of this thing to just run the light if it only happens to you every once and a while and keep the $10.00, Gas is expensive.