We had a great stay at the St. Regis, when we came to Rome in February of 2017. It was in the midst of being renovated in one wing, but the noise from construction only started after we were already up and out of the hotel. Didn't really notice it at all when we were in the room.
The decor of the room was somewhat opulently dated, but still very comfortable! The amenities had everything you'd expect in a 5* hotel. Lots of nice little tidbits showed up each day, which were much appreciated. There was also an old style lift where you have to manually open/close the metal doors to operate it. Hopefully, they won't remove this feature during the renovation work as it's really cute.
As others have commented the lobby renovation is absolutely spectacular. One downside was that due to the renovation work the restaurant dining room was closed, but they did setup a temporary space with some tables for dining service. The food was very good.
Overall, a great place to come back to after doing tourist things and the staff were very pleasant too. I'm sure it'll be even more sumptuous once the renovations are complete.