California law allows upto 90% on both rear passenger windows, but a maximum of 30% on front driver and passenger windows (and this has to include the factory tinting in the glass characteristics). Nothing is allowed on front windshield except for a narrow band across the top, if it meets certain distance and angle criterion from the driver seat. The California DMV web site has details!
For my son (he has some sunlight sensitivity) I have 90% on rear side windows and rear windshield glass. In retrospect, I overdid the rear windshield glass ... makes it a bit tough for my wife to drive my car at night. So, in her car, we did 30% on rear windshield glass, 90% on rear side windows and 30% on front side windows and a narrow 3" band across the top of the front windshield.
I agree with J-Owner - the tinting keeps the cars cooler in the sun! Get it professionally done and it will last a long time. I had my Q done over two years ago and there isn't a single bubble in it yet.
I also agree with fxjackso in that my Valentine 1 rear-facing radar sensitivity is somewhat reduced (from what I can tell), even though I did not use metallic tint - a small trade-off for keeping sun off my son.
Z