Perfect for the San Francisco experience
The Rex Hotel is exactly what you would expect a building in San Francisco to be like. The building is old, the elevator is tiny and it is in the heart of the all the action a tourist could want. 
If you want to stay in a modern building this is not the hotel for you. The elevator is tiny. The stairwell is crammed and the steps and floors are uneven. But that's all part of the charm. 
Our room faced Sutter Street and there was considerable noise every night. Everything from the traffic to rowdy crowds to the garbage trucks and street cleaners. Fortunately I'm a fairly heavy sleeper so it didn't bother me at all. The room was perfectly serviceable with a modern TV and a typical minibar. 
One of our favourite features of the hotel was the 5 o'clock wine tasting in the lounge. It was great to start eacj evening with a couple pre-dinner cocktails and sample some of the local vintages.
The staff at the hotel were very friendly and helpful. They made some great recommendations for dinners and handled all our reservation needs. The valet parking is quick and not too pricey. Valet parking costs slightly more than some of the close by lots but you can come and go as many times as you like throughout the day so if you're in and out it a few times a day you'll probably end up paying less. The only problem is that the service isn't available between midnight and 6am which meant we had to get creative on the last day to make sure the car was available at 5am so we could make our flight. 
Union Square is tourist central. Almost obnoxiously so. If you're there to shop and sight-see you'll love it. We wandered and found restaurants nearby a couple times and went a little further a couple times for something specific. Only once did we need to take a cab though as there are so many options so close by and we enjoyed taking in the neighborhood walking a few blocks.