Oh h*ll nay!! I'll give you an example:
I was in Tesco, perusing the ready meals section. What I really wanted was the mushroom risotto with double cream and truffles. But I thought, I "should" have the one from the healthy range. I took one in each hand and compared the nutritional info. The "diet" one had 100 less calories. But it doesn't stop there! It had THE SAME amount of fat, and 6x as much sugar!! So you can guess which one I bought.
In my experience, diet foods aren't up to much. They either leave me completely unsatisfied and craving donuts, or where they remove fat, or calories, they really beef up salt, sugar and sweetener to make the food taste of something. It's that kind of disjointed diet mentality that got me into this mess in the first place. Now, I eat the full fat versions. Without fail. I eat what I want, when I'm hungry and I stop when I'm satisfied. I can always eat again when I'm hungry later. Diet foods unequivocally make me hungry twice as often.
Eat as healthily as you can, find healthy foods you could eat all day long, even if it's only a few. And if you want a cake now and again, that's cool. Just not all the time. I avoid diet foods because they make me want to eat more, and crazy uncontrollable cravings are not fun!