I have been using the Premier Agent program through Zillow since April 2010 and have had great results. The first sale I closed from a Zillow call paid for the whole year. I upped my advertising after I realized how valuable this is and how well it works.
The short time that I have been part of the Premier Zillow program, I have had more contacts in the two month period than any previous two months going back 2+ years. I am assuming that with the market increasing that I will gain even more inquiries.