I wrote about the survey-taking a few months ago. The library says that almost 33,000 people took the survey, which means that it's a little less than ten percent of everyone who has a SPL card. The survey was conducted by consulting firm Berk & Associates, and it cost SPL's supporting nonprofit the Seattle Public Library Foundation $120,000. The results, which you can find in PDF form over here, are for the most part unsurprising: People want more materials, more hours of operation, and they use the library's hold system quite frequently. (Of course, the people who would go to the library to take these surveys are likely to be the most frequent SPL users, too.)
The majority of respondents said that if they had ten bucks to invest in the library, they would put most of their money into books, with A/V and online resources coming in a distant second and third. Considering that a good portion of the survey had to do with e-books and technology, that's kind of a surprise.