All things in moderation. Fast food should be meant as a treat once in a blue moon, not someone's main menu.
The biggest crime of the fast food joints is their pandering to children. McDonald's (with Disney, a partner) has been exploiting kids for as long as I knew they existed. The trick is, you get kids into the restaurant with the fun atmosphere, even the playgrounds, and they have that fun factor in their heads as they grow up. Before you know it, they bring their kids there. HFCS, or high fructose corn syrup, is added to much of the menu, which has the effect of having you not recognize you're full yet.
I advise anyone to seek out a book called "Fast Food Nation" with a TON of facts about the fast food industry. It's a pretty informative and scary book, really. A movie based on it is coming out soon.