Stayed here for a conference in May - sadly my room did not oversee the Arch, but it was literally across the street. Really nice park there, by the river. The Hyatt rooms are very clean and comfortable, and the food is good. I had a long wait to eat on the first night of my conference, when the Ruth's Chris closed for a holiday and the other restaurant was overloaded, but they left me chocolates in the room as an apology, which I thought was lovely.
I was a little disappointed at having a standing shower instead of a tub, because I would have taken a hot bath (best way to relax after a busy conference day), but otherwise the bathroom was really excellent.
It's a very short walk to the America's Center; I didn't really venture further but it seems like a good central location for business.