Decent Business Hotel, Centrally Located but....
Having lived in San Francisco and now travelling there frequently for work, I am very familiar with the city and have stayed in numerous hotels within Union Square as well as the entire bay area. This hotel is an average typical business hotel with all the ammenities one might expect: basic room, clean bathroom (larger than most in SF, as is the case with all Union Square business hotels - Parc 55, Westin, Hilton, etc), restaurant/bar, close to transportation. Whether travelling for business or leisure, the location is perfect. Right in the heart of Union Square and only a short walk to the BART, Chinatown, Muni, etc. 
I was in SF for business last week and due to the US Open, availabilities were limited so I was booked by my company's corporate travel at the Hyatt as opposed to Parc 55 (my preference for business trips). I had no complaints about the stay other than the fact that the lobby seems to always be packed with people and the hotel charges for wifi. The doorman was awesome and gets a cab for you in record time.
I would have easily given this hotel an average to slightly above average score if not for the issues I had during check-out and the week following. I opted to check out at the front desk to ensure that my personal credit card was not charged for the room (the room was pre-paid on a corporate card). I am glad I did this because they were, in fact, charging my personal card rather than charging the corporate card which was on file (I even saw it in the book along with my reservation while sorting this out at the front desk). After 20 minutes of frustration (and now being late to a meeting), the girl at the front desk finally (allegedly) got the situation resolved and gave me a virtually blank piece of paper which was informing me my balance was $0. I was a bit suspicious so I checked my pending transactions on my credit card the following day and sure enough...there was a pending $800 charge from the Hyatt. I immediately called and was transferred to the accounting department. After another 30 minutes of frustration, I was told that it was now officially resolved and my credit card would be credited. 3 days later, I checked my card again. Now the charge was no longer pending but rather, an actual charge AND my statement closing date had occurred so I was getting charged interest on this charge. I called AGAIN and finally was able to get the charge removed however, the interest is still on my statement and I have to file a dispute with the cc company all because the of the Hyatt's incompetence. 
Overall, this hotel is nothing special (no historical significance like the Sir Francis Drake, not a see and be seen place like the Clift and certainly not the best business hotel out there) but it does the job and is in a good location. The Hyatt clearly need to get a little more organized and not cause their customers days of frustration over a simple 2 night stay. I certainly wouldn't recommend this hotel based on my experiece.