I was at Werchter (in Belgium) last year. Brilliantly organised. Great value for money. Class line-up. Good public transport links (Werchter is within striking distance of Brussels, its airports and train stations). Flawless.
There's another lesser known 4 day long Belgian festival worth highlighting: The Dour Festival. Their website is here:
http://www.dourfestival.be/en Check out their
2009 lineup as well.