The atmosphere is AWESOME. I have friends at other med schools and they seem to be a little miserable right now compared to my great experience.
If you don't have maize and blue pride when you get here, you will definitely catch the Michigan bug soon after you arrive. Most of the med school class got season football tickets (our seats are really good) and we go to all of the home games together. We tailgate before the game as well. People are starting to go to hockey and basketball games now that those seasons have started. People always talk about the Michigan "family," and when you become a part of it, you feel like you are a part of something big and great. The school spirit is super strong at the med school.
As for dog-eat-dog, it couldn't be anything further from that. Collaboration abounds here. Since we are truly pass/fail the first 2 years, there is no competition whatsoever. We have a facebook page and people post helpful resources they find for other classmates to use. We also have a wiki page where classmates upload their study guides for other classmates to use. We even have access to study guides from previous classes. People form study groups and there is nothing but help going around. And this collaboration leads to great results, we rock the boards and get into great residencies.
We also do a lot together outside of school. Besides the football games, we go on food outings as a class to various restaurants around the city. There are often parties for people's birthdays. We play IM sports together (I am playing on two IM soccer teams right now made of M1s, a coed and men's team. Next month I am starting IM flag football and innertube waterpolo). Also with the facebook page, if someone is doing something (biking, blueberry picking, going to a cider mill, bar hopping, visiting a museum, seeing a movie, rock climbing, hiking, doing photography, etc.) they post about it and invite the class, and chances are at least 10 people will show up to do it with you.
As you can see, we have a ton of free time here at Michigan to explore our outside interests. Med school here is great!