The TL;DR: Get at least the sides tinted with ceramic if you live where the sun shines. It can be perfectly clear if you like, it doesn't have to darken the windows at all. If you're in a warm climate, where the temps often get above 85F or so, get the windshield done in clear ceramic if you can, and get probably get the roof done in ceramic as well. Get the roof done for sure if you often see temps above 90-95F or so.
General comments: The biggest difference you can make in the effectiveness of heat transfer is ensure that you're getting ceramic tint, not the older (carbon?) versions. It's a huge, huge difference. After that, just pick what darkness you're comfortable with seeing out of. Keep in mind that all of the windows behind the driver are already tinted to some degree. Also, your auto-dimming rear view mirror uses a sensor to measure light hitting the mirror from the rear. If you tint the rear window too dark, your rear view mirror might not dim for you. Learned that one the hard way once.
Regarding the below: I live in the Houston area. It's hot and humid here.
This car, vertical windows: If you live anywhere where the sun shines, do yourself a favor and get at least the sides and rear tinted with ceramic, even if you don't darken at all. I've had tinted cars all my life and couldn't believe how much better this is to live with. If it's legal, get the front done with clear ceramic if you're in a warm climate. Yes, it's going to cost you some money, and it's so worth it. I think I paid in the range of $270 for the side windows.
This car, the roof: This deserves its own discussion. I had the roof tinted a couple of months after I had the sides done. It was just a little too hot, even with the sunshade closed. I had it done with ceramic tint, with the lightest tint they had that wasn't clear. Is it worth the $175 just for the roof? I'm going to go with a "probably" here. There is definitely an improvement, but it isn't nearly the difference that you see with the side glass. There are two factors here: the roof glass already has a fair amount of UV protection built into the glass, so by the time the sunlight gets to the tint there's less to reflect back out. And, of course, the sunshade helps immensely when it's closed. In the end, I'm glad I did it. It does make cooling the car much easier on a sunny day - and that's what the tint is for.
If you've never had window tint, make sure when you clean the inside of the car you don't use ammonia! Vinegar and water, or even better just some lightly soapy water on a microfiber cloth will get your windows cleaner than you thought possible.
Unsolicited and unpaid recommendations for those north of Houston:
Tomball Tinters did my side windows, and they've tinted every car I've owned since the 1990's. They refused to do the roof, however. Apparently they had an issue with some water dripping into an Audi's electronics when they did a roof - and they implied that was expensive. No problem; I still recommend them. They're great.
So, I went to Performance Tinters in Conroe, who were happy to do the roof. Also great, very nice to work with.
Either one would be a fine choice.