Fast Food isn't that bad.
They still have to pass health regulations. Granted some countries have crappy health regulations on what they allow to be put in foods or to be used as preservatives (See US health regulations which allow a multitude of unhealthy additives that other 1st world countries do not).
It all comes down to how much of it you eat and how often.
If you're having McDonald's several times every week, or KFC etc.... then yes you're going to get health problems. That's providing you're eating the fatty foods, such as burgers, fries, sundaes etc which are all loaded with fat, protein and carbs and oils.
If I remember right from my Physiology class... To burn off a Big Mac after eating it you need to do over 1 hour of heavy cycling, or 35 minutes of rowing.... that's a lot just for one food item... so yeah if you're having this kinda stuff frequently and not eating healthy alternatives the rest of the time... of course you're going to start gaining weight and screwing up your health, especially if you're not exercising to the same degree of what you're consuming.
as for Supersize me... I still haven't watched it but it sounds like a dramatic exercise in filling your body up with fats and oils which would naturally wreck havoc with your system if you take too much in... there's a reason why it's in that tiny space at the top of the food pyramid.... no diet should be allowing you to consume that much oil on a regular basis let alone 3 meals a day.
and as for this 4 year old burger... dunno whether to believe it's really 4 years old, but then again I found a sandwich in a school bag from 2 years ago which was still half good