Here's the schpiel on the non-Chicago campuses that I learned when interviewing in Rockford at the end of last February (because I was interviewing at one and because it was so late in the year they really gave us an idea of what they were).
Champaign--First off, everybody in the non-Chicago campuses spends their first year here and then 50ish go to Rockford and 50ish go to Peoria for M2,3,4. The remainder in UIUC is pretty small--like 25 people I think. One unique thing about the Champaign program is it's where the MD/PhD students go, so I think that makes up a big contingent. There are still plain MD students there.
Rockford--Small class, small campus. The people I met there were very nice and enthusiastic about the campus. They have some student-house next to campus meant for studying and hanging out that was undergoing some renovations at the time so it was supposedly going to be sweet this year. Classes only run from 8-12 so I think that's the biggest difference--it's more studying on your own than Peoria.
Peoria- I haven't seen the campus or met the people. There's two things I know about this campus. Its classes run like all day from 8-5, and its match results are almost always the highest as far as matching first choice percentage of any campus (including Chicago).