I'm afraid to say that there's no actual evidence to suggest that this stuff works. The fact that it's invisible to the naked eye means that it's also pretty much invisible to cameras.... so the number plate is rendered no less visible on camera. Also, only a limited number of "scameras" are of the old Gatso variety (which flash from the rear) anymore - the vast majority these days are either Truvelos (forward facing, no flash) or SPECS average speed check scameras - which constantly monitor the line of traffic, recording numberplate details and recording the average speed. Even if this stuff was highly reflective enough to disrupt Gatsos, it would at best provide a false sense of security.
Another way of examining the validity of their claims:
In the UK at least, it is a serious criminal offence to interfere with the readability of your number plate. If this stuff actually did what it says on the tin, it would constitute a far greater crime than just speeding. Therefore, it can only be 100% legal if it doesn't work at all.
The best way to avoid speeding tickets is to improve your awareness - good observation skills and awareness of the behaviour of other drivers in front (i.e. braking when they spot a scamera) is by far the best defence against them.
There are multitude of new scameras that have sprouted up in locations that are hidden from view - illegally, I might add - in the UK, scameras are supposed to be highly visible, so that motorists can ease off the throttle in sufficient time to avoid late braking. This is actually a major cause of accidents, rendering these scameras likely to cause MORE accidents as a result. Claims of being "safety cameras" are completely bogus - "scameras" is therefore a far more appropriate term.
See
this post for more details on how scameras increase accidents, and
this post for more information regarding new scameras.
A good safety device is a "Road Angel" - GPS with built-in speed camera / speed trap / traffic blackspot database, which gives early warning of illegally hidden cameras, thus giving an early warning of any late breaking / traffic pile-up situation ahead.
