I had my windows tinted recently and have done some research before deciding on the tint.
My primary goal was to find window tint that would give me the most heat rejection. I wanted to keep the factory look so even 20% tint would've been too dark for my tastes. Percentage numbers represent the amount of ligh transmission, 5% is limo tint, 35% is legal tint in most states. You can find state window tinting laws
here.
I checked 3 major brands that offer lifetime warranty and have installers nationwide: Llumar, Formula One and 3M.
All of them use metalized tints that will not fade or change color and 3M additionally offers carbon injected film called Colorstable that doesn't have any metal in it and still comes with lifetime warranty.
Metalized tints offer the best heat rejection but it's good to check the specs before making a decision because it varies from brand to brand. I decided to go with 35% metalized tint from 3M because I liked the look and it provided a whopping 50% heat rejection. Other brands that I mentioned also offer good performance but I didn't care for their look, it was all charcoal and darker than 3M.
Initially I wanted to go with 3M Colorstable tint since I thought it would not interfere with radar detector and radio reception but after talking with the installers I decided not to worry about it and so far don't have any problems.
Installation is top notch, tint goes all the way to the edge of the side windows, rear window is done in one piece and it looks like film sticks to the dreaded dot matrix. This can be sometimes problematic and one of the installers said that he would sand it down but I didn't want to do that.
I was told not to roll windows down for at least 3 days and not to clean them inside for a month. I had a few bubles on the windows that disapeared with time.
I would recommend any of the brands I mentioned but if you want lighter, natural looking tint in 35% range then 3M is the way to go. It is more of a smoke tint with some blueish hue to it and is quite reflective in certain light. I think it especially complements gray and silver metallics. Charcoal looking tints like Llumar and Formula One look great on dark cars.
Picking the right shop is more important than picking the brand, make sure you get lifetime warranty on the tint and good warranty on the installation (mine is 5 years).
Good way to check if you like the look of the tint is to take some film samples from different shops and stick them on your windows. Removal can be problematic but some Goo-Off should take care of it.
Here's a pic of what my car looks like with windows tinted: