I had this installed. It seems to improve visibility in direct sunlight by about 60% and at very wide angles by about 10-15% In other words it works, when gauging from the driver's seat. If your eyesight is directly perpendicular to the screen, improvements are even better. I s'pose this is related to the way it polarizes light.
My biggest problem was whenever the sun was high up in the sky and rays hit the screen through the windshield. That is where the 10-15% improvement comes in, where as before, I could not see anything, now I can. Sort of. But the range of angles at which I would not see anything is smaller.
If sun is to shine directly through the front side windows (even tinted at 35% as mine), there is an improvement still, this time by about 60%.
Would I buy again ? Yes. Worth the money (i purchased the 10 inch Asus EEE-PC screen which I then cut) ? Probably not but it worked as advertised and improved the readability of my screen.