As a former resident of Des Moines, I know where to stay when I return to visit friends. And although I usually book my reservations through priceline.com to save some money (and believe me, the savings are often extreme!), we are often put in the downtown Marriott. I have never had a bad experience here—the staff is always very courteous, the rooms are spacious and recently renovated, and we have had no major complaints. If there is a downside to this place, it is the varied hours of the food/beverage establishments. The bar on the first floor is open odd hours, i.e. closing at 9pm on a Saturday night, sending us up to the Skywalk Lounge on the second floor. But if that is the worst part, you shouldn't shy away.
The hotel is centrally located in the downtown area, and although Des Moines is not the largest city on Earth, it is still easy to get lost if you don't know where you are going. We always skip the valet parking ($12/day) and park in the city-owned ramp about a block to the west (across from the 801 Grand building). In this ramp, parking is free from late Friday evening until early Monday morning, so if you are staying here on a weekend, it's a definite plus. Also, this ramp is attached to the skywalk system (as is most every other building in downtown Des Moines), so if you can navigate the skywalk system, you get free parking and don't have to walk outside! If you stay on a weekday, the parking in the 801 ramp is still more affordable than the Marriott ramp, although you have to pay every time you exit.