The only good thing about fast food is that it's really fast and inexpensive. anything else would be its huge drawback.
but from the second thought, if the food is quickly prepared, it's actually not a good thing at all. the longer you cook, the healthier food you would eat. this unlikely happens in vast majority of fast food restaurants. The idea here is to serve more people for as shorter time as they possibly can. this is also related to poor quality of the food itself, insufficient cleaning procedures or rare replacing deep frying oil. Not to mention that these places tend to hire irresponsible young people that often play and abuse the meals by deliberate spoiling or damaging them.
So I would suggest thinking twice before you're finally tempted to have a nice big cheeseburger, chips or pizza.