i think that every thing have two sides, and eating fast food is not an exception.
It is undeniable that eating fast food can be harzardous to your health because MOST of the fast food are high in calories, fats and salt, and also, they has only a small amount of fibre.
However, please note that not all the fast food are bad to our health. In fast food chains like Macdonald, Delifrance and Taco Bells, there are food like Green Salad and reduced fat milk. And if we pay attention to the calories of food items in those stores, we may note that not all of their food is high in calories, for example, in Macdonald, a regular hamburger contains less than 300 calories.
Also, the amount of calories/fats actually depends on the size of the meal we take, if we eat the "Jumpo" size food, of course we are going to take in a lot of calories, but if we just eat the regular size meal, the calories/fats/salts intake will not be so high that it may cause adverse effects on our health!
PS: i think that the most tasty fast food is the burrito from Taco Bells!