Tinting depends somewhat on your end goal. By no means are all tinting films created equal. Darker does not always produce better results. IOW, visible light transmission is not as important to most people as the IR and UV transmission...it doesn't have to be dark to block most of that.
3M's Crystalline tint series is made up of over 200 layers...non-metallic so it doesn't affect your cellphone reception, and does a great job on both IR and UV protection. It's the only one (or at least was) acknowledged by the American Cancer Society for UV protection. It's IR rejection is pretty great, as well, and the difference in light transmission doesn't have to be very much...their lightest tint is almost as effective at blocking things as their darker ones. But, they only sell it to trained installers...you can't buy the material and do it yourself.
Some people want it for privacy, but that can make seeing details out of the car quite difficult at night where your eyes are already straining...stuff starts to disappear.
I've had it installed on two of my cars, and if you apply it properly, the up/down on the windows when fresh wasn't an issue...you might want to get it a bit drier before closing the door, though. Normally, they tell you to not open the windows for at least a few days after tinting.