It IS NOT legal, but if you can get a note from your opthalmologist or even an optician that your eyes are sensitive to bright light (isn't everybody's?), it will get you off a ticket if you ever got one. This is according to a 20 year vet of Highway 1, NYPD friend. I've had them on my ML for 3 years (as dark as the rears) and never got cited, stopped 4times in this car for various velocity-related reasons and NYPD nor State trooper didn't even mention it.
HOWEVER, I don't recommend anything darker than a 40% tint. I actually felt vision-impaired, especially at night. My wife hates driving this car for this reason.
I'm doing 30% on my CLK.