We use 'Is this useful? Yes / No' feedback. We have targets from senior management to improve feedback results, but they are not getting better, no matter how many UX and content improvements we make and errors we avoid.
I've also noticed that visitors are less and less likely to submit feedback as time goes on.
So, is this just a rule for websites as they get older - does feedback worsen over time? My theory is that when the website was new, people were keen to provide their feedback – but once they had done so once, there was no reason for them to do so again, and so over time the feedback becomes biased towards those who just want to complain.