I get asked this almost every day, it seems to be a good ice breaking questions many of my customers use. For me , I tell them how it is, I don't lie , but I also don't make it negative when its been slow. I tell them its been slow, but then also tell them how Ive been trying to get things moving. I have found it sparks a more constructive conversation than whinging about how the economy sucks and sales are down.
Having said that , when its slow for us, its not that we have no customers, its just less than usual. I have not had an empty workshop in the last 3 years I have owned it , or to be honest in the 7 years prior to that when I was their technician.
My previous boss had the rule that you ALWAYS said business was booming, even when not, as they thought this false image would make them look better, but for me honesty has worked better as my customers often have input on ideas for me, as well as some good referrals on support services ( such as sign/printing and advertising companies).