Inspired from this discussion
It seems clear to me that fast food is hopelessly unhealthy and expensive compared to almost anything home cooked. I don't buy the "I don't have time" argument either, unless one is on the road between destinations. Nonetheless...
...while in college in 1984, drained of funds and willing to do nearly anything ethical to get by, I took full advantage of the McDonald's Olympic game. This is the one where you get free food if the Americans medal in the event printed on your free game card you get when you go in. I would wander by between classes and scoop up game cards left by those not as desperate as I. When the month to turn them in came, I did so. All of them. Every day. 3-4 times: a Big Mac, fries, and a coke, for a month. I didn't gain a pound, nor notice any ill health effects.
Then again, my main mode of transportation was my bicycle, and to say my metabolism was high would be a major understatement. The lesson I take from that is not that fast food isn't unhealthy, or can't be cheap for that matter. It's that one shouldn't do medical research with a sample space of one. Oh, and never eat 3-4 Big Macs a day for a month. It's likely to cause you to never eat one again.