Great stuff river and terry, and thanks agold for posting this up. Hopefully our (first years) perspectives can help relieve some stress of the applicants.

I won't repeat much of what river and terry have already said (they are much more eloquent than me, and have always been), but the one thing I have to stress is,
get your priorities right before you start school.
Once school starts, you'll be washed away by a storm of academics and activities and you'll never really have the time to sit down and figure out what is important to you in your 2-years at B-school. By the time you do figure it out, it may be too late. What I mean by this is, are you in b-school to mainly form a network to get into that hard-to-get-into-industry? Are you there mainly to learn the material very well? Are you there to learn more leadership skills by running clubs? I'm not saying that any of these are mutually exclusive (quite the contrary), but you have to figure out which one is
more important to you, and when you have conflicts (which you definitely will have), it will be easier for you to pick the thing that matters more to you.
For example, I decided before school started that as much as I like to lead and influence the direction of a club, in order to participate in all the clubs that I'm interested in, I will not run for co-chair/president of any club, just many VP positions. Academically, I have already gone through the "grades matter most" engineering undergrad experience, so I told myself that as long as I'm learning the material, I will always sacrifice my school work if I need to get a club event done or do something that will help my professional skills.
Make a mental list like that for the different things that you may face in school (best way to know is to talk to current students and learn about the things that pull us in different directions). Also know your limits. Again, this is best learned by talking to the 2nd years. I probably would not be alive right now if I didn't talk to a lot of 2nd years and learned from their "mistakes" of over-committing. Because of that wisdom, I am forcing myself to only commit to 3-4 clubs (doing VPs for everything) and just being a member for the rest. I'll still be super busy next year, but I'm pretty sure I won't be so busy that I won't deliver on my promises.
So far I've been l
oving every single day of my experience here at Berkeley Haas. I just remember every morning, when I was still working, waking up dreading going to work. But now, even though I'm even more sleep deprived, I look forward to a full day of exciting times ahead of me. Like all the other 1st years have said, the people are AMAZING. One of the reasons I chose Haas is because of the "confidence without attitude" culture we have here, and it's true. I'm still surprised by the intelligence, experience, and the amazing things that some of my classmates have done, 3 months into the school year. People are just so unassuming, as if all the great accomplishments they've done didn't mean much. The class interactions have been wonderful, full of respect and friendliness (to the point a couple prospectives doing class visits have commented about it), and no one is afraid to ask those "stupid" questions.
Figuring out which social events to go to and which ones not to is still tough, but you just have to pick a few where you feel you'll get to know the most amount of people intimately. If big parties are your thing, do just that. If small intimate gatherings are your thing, then have more dinner parties or wine and cheese nights. I feel that there's a place for everyone at any of the top schools, and the people you meet will eventually become friends that last a lifetime. I don't regret a moment being here in b-school.

P.S. There is a joke here that a 2nd year showed us during Orientation Week. 75% of the people come in as in a relationship/married. By 2nd year, 75% of the people are single... kind of scary because there is some truth to that!