I was there 2009 summer in their official program. It was nice. I didn't get too much out of it because the best part about that program are the lecture series (which can be viewed online too btw). I had to come way before their program officially started so by the time the lectures had started, I was neck deep in completing my "ROOT" macros and getting some viable results to show back in my uni.
Apart from the regular lectures, there were some superstar-lectures that you can attend. During my time one of the lectures was by Weinberg (if you're thinking about CERN, you should know this name) but it was basically a review of how QFT had progressed over the years and was way too advanced for my level at that time. Another I remember was Nobel Laureat J Steinberger on the environment\ energy solutions and stuff.
Other people in the program were mostly from the member nations, some from the Americas, Israel, India and Japan (the four observer states). They party a lot, some people try to prove that nerds can party hard as well and you have boiler room parties where you'll see people as plastered as a pre-pubescent frat house party in America.
Ahh. Undergrad days.
The one good thing about it for me, or bad I don't know the future will tell, was that I had gone in as an engineering student and came out determined to become a physicist.
Good luck.