We went to SD several years ago. Corn Palace is pretty dumb. It wouldn't be so bad if it was around all the other sight-seeing stops, but the problem is, its a fairly long drive just to get to it when there is nothing else of any real significance around it. Its really not worth a 2 hour round-trip drive (or whatever lengthy duration drive it was) to go see it. All it really is is a fairly neat building filled with overpriced merchandise you can buy to advertise the fact that you just visited the aforementioned fairly neat building.
Just my 2 cents.