I'm under the impression that proper tinting requires: Good tinting film, A sharp knife, a liquid solution for laying tint, a good squeegey, super-clean glass, and a steady hand (or 3 or 4 steady hands).
I don't think there is a reason why you can't do it yourself, but its not as easy as just peel 'n' stick. There are some companies that sell pre-cut tint pieces for your vehicle, and that can eliminate some of the hassle, but you still have to get it layed down just right, and you have to do a great job of removing the liquid underneath so you don't develop bubbles. Take a long hard look at a little rice-car who's owner tinted its windows 2 years ago and it is now covered in tiny bubbles and cracks and looks terrible, and take the time to get the right tools, do some research, and do the job right. Or pay a pro to do it.
The windows on my 300TD were professionally tinted by the PO, about 8 years ago by the receipt, and they look great, no bubbles, no peeling, nothing. If it weren't for the few tiny dings where I accidentally banged the seat belt clip against it on the driver's side, you wouldn't know that it hadn't been applied yesterday.
And yes, I'll post a pic when I get home.
peace,
sam