Great location!
We gave this four stars because of its excellent location and friendly staff. The rooms are a little on the small side and, being so central, there is some road noise. Because of this, this might not be the hotel for you if you have a week or two in San Francisco but if, like us, you're trying to cram as much in as possible for a couple of days it is ideal.
Just a little more on the location - it is, as the name suggests, on Union Square. The trams stop right outside the hotel and there are many, many places to eat nearby. There are also bus stops very close meaning it's easy to get to the Golden Gate Bridge and the Golden Gate Park (the green line is the easiest to use). I would HIGHLY recommend buying a muni pass - it allows you on the buses and trams without having to worry and works out much cheaper if you're trying to get around quickly. The hotel sell these or they can be picked up at the airport. Finally, it takes about 30 minutes to walk to the Piers - perfect!