Skip's map pretty much nails the Missouri chase description. As an extreme Southern Illinois chaser, I consider the Bootheel and Eastern AR some of the best parts of my extended home area, along with central Illinois, of course.
Northern Missouri terrain-wise is 'not great' as skip says, but my biggest gripe in the middle of that yellow blob on his map is a HUGE WSR88D hole. If you get a storm in the Kirksville-Macon area, you cant see anything below 10,000 ft on the four covering Radars(KEAX, KLSX, KDVN, KDMX).
UPDATE: chrisbray, I posted before I saw your Southern Illinois comments. It is somewhat of a challenge south of I-70, and especially south of I-64, but it's not as bad as Central Missouri until you get south of Illinois Route 13(Carbondale-Marion) in the Shawnee National Forest.