Generally speaking, “fast food” chains serve only junk foods. The majority of the food they offer is mass-produced, highly engineered junk food with very little nutritional value.
These places are often very cheap, but keep in mind that junk food costs you twice.
For every penny you save there, chances are that it’s going to cost you many times more in the future. Poor health is expensive.
Alternatives: Fortunately, all sorts of healthy fast food places have started to appear. Chipotle is one great example.