What everyone else said. I certainly wouldn't recommend wearing anything else except maybe thongs for around the camp site. We were pretty lucky this year as far as weather goes, but I went to Bloodstock the following weekend and you were dead set fucked if you didn't have boots or wellies.
The only problem I had with my boots was for whatever reason something set off the metal detectors at some airports and of course the first thing they shout at you is "TAKE OFF YOUR BOOTS."