I've been several times, including the last 2 years.  It's a good show, but a lot more local stuff there than the larger shows.  (Local dealers, smaller manufacturers)  If you're targeting the area around the Waterloo/Cedar Falls region, it would be a good spot to exhibit.  If you're thinking about a more general exposure to Iowa then the Power Farming show mentioned by the others is the place you need to be.  From attending both, I would bet that you could get 10-20x the traffic  at the Des Moines show compared to the show at the UNI Dome.  I'm sure that the exhibit cost reflects that.

Mike