Nothing great, but if you're careful about your choices the occasional fast food won't kill you.
But take a careful look at the nutrition info. For example, at BK:
http://www.bk.com/cms/en/us/cms_out/digital_assets/files/pages/MenuNutritionInformation_April2013_1.pdf
Asking for a whopper without mayo cuts 160 calories and 18g of fat. Better still, the grilled chicken w/o mayo, veggie burger w/o mayo or fish sandwich w/o tartar sauce aren't terrible. All 400 calories or less and less than 10g of fat. They still have tons of sodium though so it's not an every day thing.
Skip a drink and keep some fruit or a bag of pre-washed salad at home and you won't have to get fries or soda (just empty calories and fat).