I went 3(?) years ago and hated it. I've been to something like 10 festivals and never left one early, but we left Bestival on the Saturday. The horrific weather (torrential downpours for a good 6 hours straight on Friday - rain so strong you didn't even want to leave the tent for the toilet!) was a factor, though, but not all of it.
If you're coming by public transport, pack reeeeeeeally light because the site layout is sadistic - the entrance is at the top of a really steep hill (think mudslides) and the camping areas are a significant walk away, and back up another hill. And it's almost impossible to get back up that initial hill if it rains.
I hope others come along in here with good stories of Bestival because I don't want to be a downer but our experience was just SO BAD that we swore off all British festivals after that. Too much money for a miserable weekend full of horrible people (no joke, the Samaritans were the only people who smiled at us the whole time we were there). We came home and had our own "festival at home" instead and had a brilliant time.