Surprisingly good
I booked five nights at the St. Regis before reading some of the negative reviews. Naturally, it made me wonder if I should rethink my plan to stay there.
But you know something? It turned out to be a surprisingly pleasant place -- to the point where it sounds as if the negative reviewers were staying in a completely different hotel. 
At check in, I asked the desk clerk (Ray?) to give me a quiet room, away from the elevators. He searched the database and found a great one -- top floor, facing the back, recently renovated, very comfortable, and extremely quiet. 
Apart from the occasional sound of a distant door closing in the hallway, I heard no other guests during my entire five night stay. The air conditioning worked fine, even in the midst of a brutal heat wave.
Some of the lobby staff were very warm and friendly, others somewhat cooler, but everyone did their job. I didn't bring a car (I used Detroit's bus system to get around), so I have no thoughts about the valet parking.
As for the complimentary continental breakfast? It was nothing fancy (juices, coffee, cereal, bagels, muffins), but it got me going in the mornings. And the lady who set up the peaceful little breakfast room each day was very nice. 
The neighborhood isn't downtown but further north, in an area called New Town. It's a mixed bag of government office buildings, streets with well-tended houses, streets with abandoned houses, a few fast food places, plus empty storefronts, old industrial structures, and vacant lots. Pedestrian traffic is very low, as it is in most of Detroit. By sundown, the area is deserted.
Since I live in a much larger city, none of this was beyond what I've encountered at home. But I'm sure that many life-long suburbanites would feel uneasy (although downtown Detroit can be sketchy as well, especially at night).
My final verdict? I found the St. Regis to be quiet, clean, comfortable, and a very good deal for the room rate I paid. I'd definitely stay there again.