Really, really REALLY disappointing
If you do not read this entire review, please read this: DO NOT STAY AT THIS HOTEL. OUR ENTIRE FLOOR HAD A STRONG MUSTY SMELL. THE STAFF IS UNINSPIRED AND THE RESTAURANT IS NOT WORTH IT.
I love old hotels (see my reviews of the Palmer House in Chicago and the Pfitzer in Milwaukee). I was truly excited to be staying at a landmark hotel such as the Congress Plaza, until I showed up.
It started with the valet parking. The hotel mentions on its website that it has valet parking. When I think valet parking at a downtown Chicago hotel, I expect to be able to pull up to the hotel, unload my luggage to a bellman and just enter the hotel. Not here. The parking entrance is on Wabash, a block from the hotel. After arriving after 11pm (and a six hour drive) I was not thrilled to learn that I will have to lug my luggage almost a full block to the hotel.
Check-in went alright. The gentleman at the front desk was personable enough. My wife and I were given room 452. Upon exiting the elevator, we noticed a faint smell. We did not think much of it at the time. We looked at the room directional sign to see which way we needed to go. We were told we were in the north tower (to the right) but the sign showed an arrow to the right for rooms only up to 438. To the left showed rooms going much higher. We preceded to the left. After walking through a maze, we came to rooms 451 and 453. Now, part of this is on me. I had not realized that all of the rooms were odd numbers (it was late and, again, the sign said the other direction only went to 438). My wife called the hotel from her cell phone and was told we needed to go the other direction.
As we walked down the hallway to our room, that faint odor became more intense. I hoped it would go away upon entering the room. It did not. The odor was a strong musty smell. We were only there for one night and we were too tired to move rooms.
The room was not well lit. It was hard to tell if the room was clean because of the lighting. The carpet was horrible. I did see some kind of insect. It could have been a German cockroach (but it seemed too small). The bed was awful. I like a firm mattress. It was firm but at the same time, we rolled to the middle when we laid down. The pillows were the worst I have ever encountered at any hotel. 
Breakfast at the hotel restaurant is forgettable. I expect high prices when dining in downtown Chicago. I also expect above average food and service. Neither occurred. Our waitress made several mistakes when bringing our food. She also handed the plates to us. She did not set them down on the table. She handed them to us. Maybe I am being a bit of a snob here but really?!? Water and coffee pitchers are left on the table. There were at least four wait staff personnel at the restaurant with just about as many patrons. The wait staff congregated in a corner and seemed more interested in socializing than taking care of the customers. The food had zero flavor.
If there is a positive, it is the location of this hotel. You are close to a lot of great attractions and fine restaurants. The problem is that there are too many other hotels in the area that one may stay. You may pay a little more to stay elsewhere but I cannot ever recommend this hotel to anyone in good faith.
Upon checking out, the front desk clerk never bothered to ask how our stay was. My guess is she already knew the answer...