I stayed here for business in mid-July, 2014. Two nights.
I needed a place near a conference I was attending and while there are cheaper alternatives nearby, the Park Hyatt really delivered on all fronts. The room was beautiful, with a tremendous bathroom that had a unique sliding door pass-through which almost let you reach your bed from the large bathtub. I know it sounds odd, but it is really interesting, and if I were a bath person, would have been fun to try.
The bed was large and comfortable and had great controls over the window shades (one is a blackout shade, the other is sheer). The room was quiet and very spacious.
The workout center was fairly large for a hotel and was not crowded. Chilled towels, fruit-infused water and an attentive staff were highlights.
The location right near Michigan Ave was perfect and I was able to walk everywhere.
Last, the lobby follows recent trends of being "atmospheric" rather than focused on service, but I found all the people to be friendly and helpful -- from the front door, to the desk, to my smiling turn-down lady.