Best Location in SF but Dated
A year ago, when tickets for the US Open Golf Tournament first became available we booked our room at the St. Francis. We chose the hotel for the location and charm of this grand city landmark. 
A the time there was a special and one could upgrade to a small suite for what seemed like a modest increase in price and the thought of the kids having their own room was too much to pass up. Nearly a year later, I decided I better verify the reservation to make sure nothing had gone wrong. The online system said the details were unavailable and that I would get a confirmation email a bit later. 
10 Minutes passed and the email I received surprised and pleased me. Instead of a junior suite the email confirmed a "Deluxe 800sf Corner Suite with views" plus a second room for the kids at no additional charge. Wow, I thought this SPG program really pays off. Everyone was excited to have such a great room.
Now anyone in management knows it is important to deliver more than you promise, except perhaps at the Westin St. Francis. Unfortunately, we checked in to find exactly the room I had originally expected: two rooms linked together. One was a living area with a murphy bed and the other a standard king room. I shouldn't complain, it is what I reserved but our expectations had been inflated.
One thing did live up though: the view. It was a corner room on a high floor and you really could see half the city from there. The location was perfect too. Union square is at the heart of everything. 
The room itself was poorly laid out and dated. Because it was really two rooms, much of the square footage was taken up with a second bathroom we didn't need. The decor was bland and the ceiling was like cottage cheese. Not what one would expect in the "Modern Tower."
The staff was great though we never asked for much. The lobby was beautiful, with the charm one expects from an old city hotel. 
As others have said, if you want the location this is a great option. If however, you want a beautiful room there are better options.