I went in 2000.
Despite the snow and the cold (it is a WINDY cold) on the outside of the Hilton, what goes on indoors is truly a BLAST!!!!! You will see and hear some really good groups. You will meet some very important people (bring a camera - I met Alfred Reed, Johan de Meij, Frederick Fennell, Jan van der Roost, Philip Sparke, David Holsinger, Stephen Melillo etc. etc. etc.). You'll have fun going through the exhibition hall. I recall buying a number of wind ensemble CD's that you won't find anywhere else (don't forget to BUDGET your money). Keep your eyes open for free stuff too. The conference is mainly attended by music educators from all over the world.
It is not uncommon to find high school groups performing, much less middle school groups. There are some really superb ensembles.
Have fun and definitely meet some new people!
http://www.midwestclinic.org/conferenceinfo.asp