I know what's in those sodas, and high-fructose corn syrup makes me feel ill anyway. The stuff is horrible for your metabolic system. In terms of sugar and the ability to process it, it's like drinking from a firehose, so yes, it's likely responsible for the explosion of type II diabetes.
If I have to eat from a fast-food place, I'll get something like a grilled chicken without the "sauce", which is cottonseed oil and corn syrup and adds about 400 calories, and an unsweetened iced tea.
The tea has zero calories. The grilled chicken is somewhere around 200 calories without the nasty mayo sauce. The bun, if you eat it, is probably 200. I don't eat the whole bun. The buns are made as cheaply as possible, and use corn syrup as well.
I like hamburgers, but only flame-grilled with a higher grade of meat than the fatty chuck those places use. It also seems like they're trying to kill people or something with the insistence on slapping a giant 300+ calorie 20g of fat blop of mayonnaise sauce or pesto on everything!
(BTW, look at the calorie/fat content of the sandwiches from the "fast casual" places like Panera and Atlanta Bread Company.)
I wouldn't overdo "diet", either. Aspartame waltzes right across the blood-brain barrier membrane, and they're still not entirely sure what it does once in there.