We went through this on a local street racing board awhile back. There are actually quite a few systems out there. Generally all they do is extend the length of time that a green light stays green, and on some systems, if the light is already red, it shortens the amount of time it stays red.
The 3M OptiCon is one of them, and I think it's the oldest. It's basically a plain old strobe light, and in many places ANY strobe light will set off the system. I've read about problems with the strobes on top of school buses messing up traffic patterns.
There are others which only respond to strobes at a certain frequency, and there are programmable systems that only respond to strobes in a certain pattern. All of the newer systems rely on a coded infrared signal.
They're also used in some large cities to help the city bus system stay on schedule. Buses that are running behind schedule can extend the green lights until they're in the right place at the right time again.
All in all, it's a crapshoot that whatever you buy is going to work with whatever is installed in your city -- if anything is installed at all. On top of that, they're intended to help slower-moving emergency vehicles (fire trucks, ambulances, etc) get through heavy traffic. If you search around you'll see that cops generally don't find them useful because they don't react very quickly.
It's probably not dangerous, but it certainly isn't going to help "speed demons" and these cheap bought-it-online things sound like a scam.
