We stayed here at the end of August for two nights, and all and all it was a very pleasant stay. The location could not be better - the Arch and Museum of Westward Expansion were right across the street, and Busch Stadium is 4 blocks to the east.
Our room, a basic one, was huge and well-appointed, with a mini-fridge, a spacious bathroom, and powerful air conditioning well-suited for a hot day in St.Louis. Concierge service was hit or miss - they were always helpful, but some seemed harried and uninterested. We were lucky to get a Hotwire deal that got the price of the room for under 100 bucks, but after checking the prices, we found that rooms are regularly available for 100-120 a night, a great value in comparison to the prices in other cities.
The only gripe I have was that the services in the hotel were overpriced. ATM fees, parking, Internet, the bar, and the gift shop were all way more than one would reasonably expect, particularly in light of the price for the room and the surrounding area. Nonetheless, the Hyatt provides five-star service at what would be a three-star price in most other cities. Recommended enthusiatically.