I haven't visited any fast food outlets that were dirty,never mind filthy.
I have been to a few with surly help,though. Which is surprising,when you consider they are being paid up to 16 bucks per hour to serve food and make change.
On the other hand,the semi-local McDonalds has the friendliest help I have ever seen. They even bring your food to whatever table you are sitting at. This is big for me,because I am having hip and lower back problems.
Not only that,but they are friendly and smiling,and ask is there anything else they can bring you when they come.
The closest Wendy's is about 25 miles away,and when I go there,there is always a line of cars backed up at the drive through lane,and a line of people standing inside waiting to order or pick up there food. The wait staff are so busy taking orders and handing orders to customers they really don't have the time to do much more than smile and say "Thank you".
They,and a local Hardees that I sometimes go to that have some surly counter help that have serious attitudes are the only fast food joints I visit. Even then it is usually a once or twice a week thing.