Been twice, and am going again this year. As a middle/high school band director, there is simply no better conference for our field in the U.S. I have been to IAJE multiple times as well (and am also on the executive board of my state's IAJE Unit), and enjoy that as well. To me IAJE is more of a "hang" and a place to catch some amazing professional performances. However, it is way expensive (almost $300) and generally light on clinics that I have found helpful. The state level conventions in my state are pretty lame as well, so Midwest has been a Godsend.
Midwest has so many amazing clinics going on that you will have to pick and choose and will always miss out on something. The performances are stellar as well...I have always left there a better teacher and musician, and I have decided to make it a yearly pilgrimage from New Jersey. It totally energizes me to take my teaching and playing to a new level.
Hope this helps.