We just had a last minute two day award stay at the St. Regis (one night was a FWN from the current promotion). We booked the day of arrival and were upgraded to an Imperial Room facing the square (3rd floor) with the fountain on the best side of the building.
We really loved the stay (we preferred the St. Regis to the Eden where we've stayed before). Staff and service was excellent, the upgraded room was not a suite but was wonderful and very much appreciated.
We did not find the service stuffy or overly formal at all, the St. Regis made us feel very comfortable despite coming from a week on the beach in Liguria and being informally dressed. They went out of their way to make us comfortable.