Great thread.
The surveys are helpful in many regards to get a feel for exactly what your customers think of you. Much like sending out a holiday card at the end of the year, or sending out an anivarsary card at their 1 year anivarsary etc.
We send out a brief survey 30 days after signup, then 90 and 180. We then send out a survey very 6 months and the response is very good. While not all positive, that's exactly what we need. We encourage our users to write any feelings directly to us, or put it on the survey so management can deal with the issues. We have a very large customer base that have been with us for 5 years or more.
Customer service is one of the main things that separates hosting companies. Knowing what your customer thinks of you, and how to improve and continue relationships with them is vital to the survival of any business.