I definitely think the 80% diet 20% exercise is a good ratio for losing weight.
It is easier to create a 500 calorie deficit by eating less than it is by exercising more. Many people, myself included, over-estimate how many calories we actually burn while exercising. Those exercise machines at the gym? That estimated calories burned is a myth (and one I think they won't try to fix...after all, if the machine tells you that you burned 700 calories, and you go eat a 700 calorie cheeseburger, you're eating more calories than you really burned, which keeps them in business cause you'll keep coming back to use the machine). I walked close to 3 miles yesterday (measured by a pedometer), and only burned a little over 200 calories.
Exercise will help create a bigger deficit, plus also help tone muscle to give you a better looking body (and muscle burns more calories than fat in a rested state). But it's harder to get that bigger deficit to lose weight by exercise alone.
After losing the weight and reaching maintenance, however, I feel like exercise is one of the more important steps to keeping that weight off. Staying active helps keep us focused on our health, gives us a cushion of calories that we can use to indulge every once in a while, etc.
This is all my opinion/experience, of course
I have watched a few documentaries, and read a few articles that talk about the same things. One of my favorites is
part two of The Weight of the Nation (the "episode" is named choices, which I linked in case you're interested). to me, the information makes sense. I'm sure there are others out there who may disagree or who have other viewpoints and experiences, but this one works for me