I didnt sit in on classes at Kellogg, we went through set up classes during DAK though and the professors we had were great. The ones I had werent cherry picked courses that were the most popular according to some students...they were professors who were free and willing to do it. One was a finance class which was surprising.
Personally I didnt find classes all that helpful. The one I went to at GSB with Rhyme I would honestly say was pretty bad, students actually said it was a bad representation. Tucks was odd and not that good. MITs was the only one that I really was wowed by but everyone said it was probably the most popular course people took and the professor was extremely popular and always filled up. I think interacting with students was more valuable, try to set up meetings with people at the school. Heck just sitting in the main gathering areas and people watching can give you a better sense of the school. At every school you will get great professors and interesting classes...however, not every single class in all courses taught by all professors will be great.
Usually schools put you in classes they have room and those usually arent full for a reason. At MIT I had to carry a chair into the room to have a place to sit at all the others there were plenty of empty chairs and it was clear that the MIT class was extremely popular for a reason. If you visit in the end of Sept or early Oct you will be sitting in on the first class or two of a course and the students may be just as lost as you and thats not really an accurate reflection since they have just started and are not back into school mode typically.
Try to get ahold of students ahead of time for visits. There are several students on here who are are finishing up their 1st year who might be willing to help show you around if you PM them. Next fall if you are visiting, there are gmatclubbers at every single school, and this place is all about paying it forward. We got a lot of help from people so there will more than likely be someone at the schools you are interested in willing to help. Obviously, Kellogg, Chicago, Duke and Ross are heavily represented so your chances will be better than at Stanford where its just Terry (the curse breaker). I mean with 12+ people from on here at some schools you will be able to find someone willing to show you around. Heck, it might make sense to have an official gmat club visit day at some of these popular schools.
I think my interactions with students at most schools was extremely positive, they were all in your shoes at one point and know what its like. HOWEVER, you need to remember that there are 5k+ applicants at some schools and the co-chairs of the consulting or IB clubs probably get contacted by applicants many times a day. It may make sense to reach out to other folks. If you are a minority go to the group for that ethnicity. If you are from another country and there is a club for it contact them. If you are gay, talk to those groups. If you have an interest in the environment contact the social impact club. Dont be offended if the president of the consulting club doesnt respond since it probably is getting lost in the shuffle with the two dozen other wannabe consultants who email them each day. If you find someone with a background similar to yours or from the same random undergrad definitely reach out to that person since the will probably be your best bet.