A 13.5% survey completion rate when you offer no incentive for filling it out beyond the personal satisfaction of giving you their opinion is a fairly decent response.
We got a higher response rate than that on our most recent customer survey, but we were surveying people who had bought a couple of specific products from us already, and we offered each of them a free accessory item (related to their product) in return for filling out the survey.
Personally, I once filled out a survey that promised the first so-many-people to complete it would receive an iPod. (I managed to get in quick enough -- still have & use that iPod.

) I have filled out surveys for money, for bonus points that could be exchanged for merchandise and for downloadable white papers or free copies of the survey results (which were going to be offered for sale to the public at a fairly high price).
Rarely, if ever, will I even consider filling out a survey without some incentive. And even with an incentive, if the survey goes on too long or starts asking me stooopid questions or otherwise seems to be unnecessarily wasting my time, I'll skip out in the middle with no qualms. Unless it's a really darned good incentive...
--Torka