Seth Godin breaks down the difference between a survey and a census and why you would choose one over the other (or whether you should be conducting them at all).
Here's a simple test I do, something that has never once led to action: In the last question of a sloppy, census-style customer service survey, when they ask, "anything else?" I put my name and phone number and ask them to call me. They haven't, never once, not in more than fifty brand experiences. If you're not going to read the answers and take action, why are you asking?
Source: Seth's Blog: Survey questions