We've just enhanced our website surveys so you can now install pop-up customer feedback surveys or a little tab (take a look at https://sightmill.com/tab_nps_survey).
We've found that the pop-up survey gets great response rates (normally around 10-15%) but interrupts a visitor journey. A tab survey gets just 1-2% response rate but is unobtrusive and doesn't impact the user experience. Depends on customer need, but we can now serve both without changing the tracker code - all via the dashboard.