It's much darker than factory. My state is really cracking down on tint for some reason. State limit is 35% on front windows, with a 2% margin of error. I don't see or hear of people getting pulled over for dark tint so much - instead they are cracking down on the inspection stations. The shop I always use will not make an exception on tint. A friend of mine is a DMV enforcement officer and part of his job is to go to inspection stations, under cover, and make sure they are playing by the rules. He also told, though is he not a patrol officer, his boss has issued him a ticket book and told him to start writing people up for illegal tint. Not worth the hassle.
The reason for 40% is that most factory windows have some level of tint in them. If you put straight 35% over them, the combination makes them illegal. I ran into this with my daughter's last car and had to go have the tint ripped out and replaced with 40% before it would pass. And, with the other dark glass, the doors shut, and the sunroof shade pulled closed, it looks and feels much darker than it actually is, and is still legal.