Very good, but.....
The hotel is in an incredible location atop Nob Hill. Some people complain that it is too far from everything, hard to get to etc... Nonsense, could not be in a better location and, at a max. 10 minutes walking from Union Square. Even better, it is away from all the crouds of tourists and noise. 
The hotel exterior and lobby are just breathtaking. When you first come in, one trully feels as a 1940s delegate for the creation of the UN. In addition, one of the corridors leading to the lifts was chocka block with photographs of presidents, royals, dignatries, famous people etc.... It really gave gravitas to the place.
The room was also very, very nicely appointed. Spacious, big TV, lovely vues (we were in a Signature tower room) overlooking the bay, and excellent bathroom. The bathroom was also very big, had good ammenities and was spotless.
The first night we had dinner in the Tonga room, and that was simply just a blast (food was excellent also - however, be conservative in what you order, the portions are massive!). Island Groove - the in-house band were fantastic. People spotting other tables, some people were having the time of their lives!
The concierge was also very helpful, and called two pre-booked restaurants for us that required re-confirmation of our reservation. 
However, there were a few negatives to this stay that made it not to be the 5-star experience one would hope:
1) Windows had no double glazing - meaning you did hear quite a lot of noise from the road. At night this was OK, as Nob Hill is a very quiet zone at night. However, whilst we were unpacking the afternoon we arrived all we could hear was the "ding, ding, ding" of the cable cars and the cars whizzing past.
2) Althought the Tonga room was BRILLIANT, I found it a bit cheeky that even for hotel guests a $7 cover charge per person was charged. It is not the waiters' fault, but I was very tempted to leave a 0% tip.
3) But the worst downer of it all was the undercurrent attitude, which rightly or wrongly, we both felt coming from some staff in the hotel. During our 3 night stay, we never quite shook the impression that the staff thought of us "as not quite good enough to stay at their property". This was highlighted at check-in, where we were left standing whilst at least 5 members of reception were too busy chatting and laughing amongst themselves. Once check-in had started, what appeared to be a very important doctor came into view, and without any apology or anything from the receptionist, he left us there whilst he greeted and did other things for the aforementioned doctor with loads of "doctor this" and "doctor that". Finally, to cap it all off, the receptionist then proceeded to tell us how he had been privately educated at some posh school. There was also never any effort to appear helpful. Although they new we were British (from our passports) they never offered to give us any maps, transport tips etc.... That was OK, as this was our 2nd time in San Francisco and had a ton of maps and travel guides, but the intention is what counted. 
All in all, we had a very good 3 night stay at the Fairmont, and I would recommend staying here (would definitely stay here again myself). However, this is in spite of the staff service, not because of it.