Never been but was thinking about it a couple of years ago, we never got organised enough to book it though.
Alongside the festival you have all the Off parties running around the city in the week before, the choice is a bit mind boggling and probably every major label and DJ is out there at some point.
If I had unlimited time and money I'd spend the whole week there and combine 2 or 3 Off parties with some sightseeing/food sampling and 1 or 2 days at the festival. Don't think it'll happen now though, one day is enough for me these days.