I've attended the Roskilde Festival for 4 years in a row now, and I'm going next year too. It's great fun - lots of drinking, happy and interesting people, great music and you get to be disgusting and all trashy.
Roskilde is undoubtedly a significant highlight for me each year, and going to music festivals just opened up this door into an entire new world. Everybody needs to attend one at least once in their life.