Fast food, for the most part is not very healthy food. However, sometimes you're in a rush and don't have the time to prepare a meal. In these cases, fast food is a quick and convenient solution -- albeit one that can become an expensive habit unless you take advantage of sales/deals.
The key, as with most things in life, is moderation. Don't eat that fast food too often, and you probably won't suffer any of the long-term negative effects that are always being hawked in the news.