Since I'm back to work from my JavaONE "vacations" :-), I'm constantly asked whether it was worth to attend to JavaONE, or not. Actually it is impossible to answer the question in general - the answer is highly dependent on your motivation. In case you are constantly reading blogs, JSR-specs and books, the majority of the technical sessions will appear as "common sense" to you. However, I choose some topics, in which I'm less interested in general - and learn so a lot. The technical session are more formal and "unidirectional", however BOFs are really great. I had the chance to discuss some clustering techniques with openMQ, Glassfish engineers and discuss new features in the EJB 3.1 spec. Even at the day "zero", the "Glassfish Unconference" was less spectacular, but you could directly ask specific questions to engineers and discuss issues and future directions.
However, if you expect to learn bleeding edge technologies in the General Sessions / keynotes, you will be probably get disappointed - it's better just to enjoy them. E.g. the announcement of Java FX wasn't a huge story, but the Ken Russel's presentation ("dragging out applets") in the first technical session just afterwards really was.
For me JavaONE is the unique opportunity to associate an email-address with a face, as well :-).
JavaONE is known for the parties as well - however they are mostly parallel to the BOFs. Even the After Dark concert was concurrent to some BOFs. So having fun, or learning a lot - it is your decision, however in both cases: Sleeping will be optional.
From my perspective: it was worth - it is actually the one and only conference which I fully (the whole week) attend. See you hopefully at JavaONE 2009!