I fill them out at every opportunity. I figure that getting my name in the hands of the marketing folks at my hotel chain of choice is a good thing to do. For some of the resorts (Maui and Kauai for example) I think it is good to compliment the property if you intend to go back. My wife and I returned to Maui a year after our honeymoon and they were waiting with leis at the desk, and a bottle of champagne and chocolate covered strawberries in the room. We had ordered the strawberries on our honeymoon, so someone was paying attention. I doubt it was the survey, but they can't hurt either!