I promised a report when I returned - thanks to jayer, Delta Hog, and wheresTWA for their info and suggestions.
I had 2 stays here in April. Took the Metrorail from the Airport to Union Station each time with no problems. Easy connections and short walks on both ends. There is a big difference between the rooms in the historic 1880's side of the hotel which has been renovated and the garden side which was built from the ground up in the mid 1980's. The garden side is actually inside the mall under the roof of the train shed.
Check in was smooth, I got my G2 and 1w bonus and they printed out boarding passes for all my flights. As a Diamond I recieved Regency Club rooms (412 and 327) on the historic side of the hotel. The rooms with windows facing the fountain and downtown have the best views. 412 was on this side, while 327 looked out on the roof of the mall and train shed. Rooms had Grand Beds, high ceilings, windows that opened, and throwback classic decor. I was just down the hall from the Regency Club which was open daily for breakfast and happy hour (weekends included). Breakfast was typical RC fare, evenings had cheese, fruit and raw vegetables with a hot appetizer. Refrigerator with waters and soft drinks was open 24 hours as was the coffee machine.
As stated by others, this is a historic property and the overall feel of the place is different than many Hyatts - I enjoyed it and would return. It reminded me of the Hyatt Printers Row in Chicago which is sadly no longer a hotel. The Union Station lobby and public areas were outstanding and my rooms were very quiet. There are under 200 rooms here so I never felt crowded or rushed. The historic side has about 75 total rooms (about 30 each on floors 3 and 4 with a dozen of floor 5). The rest are on the garden side. All floors and elevators require key access so noise is not an issue.
The Union Station Mall that is connected to the hotel has a food court, Houilihan's, Hooters, and more.