I interviewed last year. For me, this was the most important part of the interview experience. It told me much more about the program than the formal interview schedule the next day. Residents more relaxed, people having a good time, you can really get a feel for the culture of the program and how the residents really feel about the program. Good food, alcohol (though not too much of it), and good conversation - all on the house, what more can you ask for?
Regarding alcohol, I'm in the camp that you should have a drink or two and enjoy yourself, but definitely don't get drunk. I know some people advocate not drinking at all, but I plan on attending dinners in the future, and I know it'll help if I see an applicant who can have a couple drinks, be themselves, make good convo, and just all around seem like a person I'd like to have around.
My 0.02