I agree with you on this one. A lot of my friends from high school went to Purdue, so I went down there for the ND/Purdue games while I was at ND ('05 and '07 games). The fans were not only rude, obnoxious, and confrontational, but also criminal at times. I saw an elderly couple wearing ND gear get harassed and
spit on by a group of PU students. F*** the Irish chants could be heard throughout the day as well. I told my friends about this and he told me the student body, in his words, "pride themselves on being douchebags."
For those of you that don't know much about Purdue gameday tradition, they have a thing called Breakfast Club. Students show up at the bars in costumes at 5-6am and drink until kickoff. Gamedays are more about drinking and being an a**hole than winning a football game. I can't say I blame them since they do not have much to look forward to as a PU fan.
USC fans are fair-weather fans, but are surprisingly easy to get along with on gamedays. I haven't had any negative experiences with Michigan fans, but they do seem a bit delusional and arrogant about their team. Michigan State gets my vote for 2nd worse behind Purdue because they have a very similar, classless approach to football games and are insufferable when they beat us.
As far as ND goes, all I've heard from visiting fans is how vibrant, pleasant, and friendly the gameday atmosphere is on campus. The biggest knocks against us are the many delusional fans we have, as well as the fair-weather/bandwagon fans, which most of us true fans hate as well.