If you get the plate cover type they work, yes. They are basically like polarized sunglass lenses.
You ever use polarized lenses? Know how if you look at something back-lit one way you can see fine, but if you tilt your head you can't see see it?
The camera blockers work the same way; if you look at the plate from behind the car you can see it fine, but from overhead or from the side the plate is shaded out and unreadable.
Now; to the other post above; these types of plate covers are illegal almost everywhere so use at your own risk...
The better defense against red-light cameras is to simply not run red lights, yes?