Generally speaking it's "unhealthy" in the sense that it is extremely dense in calories, sodium, sugar, and fat. That makes it very difficult to work into a balanced diet. I'm using the term diet as it is meant to be used, meaning simply what you eat, rather than the meal plans that most people call diets.
If it had an equivalent amount of protein, fibre, vitamins, and minerals we would probably call it a superfood along the lines of pemmican.
Reactions to McDonalds are much the same as experienced when many people try any other food they are not accustomed to. Assuming you mean physical reactions such as diarrhea.
Some people have concerns over the source, treatment, and preservatives of the ingredients used but the fact of the matter is that corners need to be cut to get such a low price. Sure you can make much better burgers or fries, but they generally also end up costing double or more.