I think if I was put on the spot to give feedback then I wouldn't be constructive even if I saw there was issues as it could be taken offensively. We did an anonymous feedback a few years ago for our first festival. No names, no emails, they just filled in the form. It didn't make a lot of difference to how we do it now with their names on the form. At the weekend event, people just want to enjoy themselves and at ours at least, they don't want to be hassled with feedback questionnaires.
Although, next year I may offer up an anonymous feedback/suggestion box. Leave some pens and paper aside for people to fill in themselves and drop in a box. Might even give them the form as they arrive at the festival with their goodie bags. I think that could help us get more genuine feedback which is what we are interested in.