Just got back from a five night stay at the St. Ermin's for a short vacation trip (I usually stay at one the of Radisson properties in the West End), and it was a quite a nice stay. Most important, all the hotel staff was exceeding helpful and friendly -- a top priority in my book. I'll also echo the comments from others that the hotel lobby looks amazing. It's almost too nice; when friends stopped by to meet me at the hotel, they probably thought I was staying somewhere incredibly extravagant.
As a Gold, I was upgraded to an Executive King, and it was a pretty nice room. The size seemed about standard for a London room, very clean with comfortable furnishings and nice bathroom. The view wasn't anything special, but there was good light and the room was very quiet.
Since this is an Autograph property with no lounge, I was also given full breakfast buffet each day in the hotel restaurant. The setup seemed a bit makeshift, but the service and food was all good.
The location is pretty good -- right by Scotland Yard, a nice, short walk to the St. James's Park tube station, and a pleasant walk to many London sights. After staying in the West End so many times, the neighborhood seemed almost too quiet, with virtually nothing in the area open late or on Sunday, but a bit of peace and quiet isn't a bad thing either. All-in-all, it was a very nice stay and I wouldn't hesitate to return on a future trip.