A mixed bag
I found the US Grant good, but not great.
They make a great play of their history - to be honest I found this a little over the top, but fair play, its a historic building in San Diego with a storied history.
First of all: the great - the staff were as as warm, courteous, accommodating and welcoming as anywhere I've stayed (which to be fair, is a very long list). Stephen on front desk was particularly helpful as were others.
The rooms are well appointed. I stayed in a couple of different rooms, a standard room and later, was upgraded to a landmark suite. Great showers, slightly fussy washbasins, thick robes etc. But dig a little deeper under the surface of those luxuriously appointed rooms and you will find issues. Lots of broken curtain rings, and the bed I had in 208 had a partly concealed metal bar sticking out under the valance - I found this out when I cracked my shin against it rather painfully the first evening in there. Little things like this do take a little away from the overall impression of luxury. If you're going to sell a hotel on such an image, then details matter. Beds shouldn't be in that condition. 
I think room choice is important. While the amenities are of a high standard, there are areas of the hotel that are better than others. The suites I saw were either located below an incredibly noisy kitchen generator that I was told ran from 5am to 11pm at night (another legacy of an old building - rooms ending in 06 have this problem in particular) or were tucked away in a corner by an elevator with limited natural light and on one side, literally a view of another wall, 10 feet away.
In fairness to the hotel they were almost at capacity and I would imagine there are plenty of rooms and suites with better views, less depressing ambience and less noise. Nonetheless, take care in your choice, and if you're not happy, ask to see another room as they vary dramatically.
Generally speaking the lighting is also very gloomy, which isn't helped by the general dark brown colour scheme of the communal areas. There is also a rather unusual vaguely musty smell in the air. No doubt another quirky legacy of an old building! The hotel is also popular with pigeons who made it their business to land on my window ledge most days. 
The room service breakfast was fantastic, in keeping with the generally very high service levels. Wi-fi was adequate, no huge issues. The fitness centre was below par however. Tucked away in (another) gloomy corner of the maze like lower lobby it had no windows, limited air conditioning and very limited equipment. For a five star hotel this was definitely below par.
So in conclusion, great personal service - I cant remember a single occasion where I was not addressed by my name - some luxurious touches but the curiously depressing ambience, lack of light and 'quirky' room layouts may not be to everyone's taste. Would I stay here again? I'm honestly not sure. I checked into the W, just a few blocks away right after my stay at the Grant and its a very contrasting experience and to be fair, sells itself on different qualities. Its not all great either, but definitely highlights that to stay at the US Grant is a mixed experience for a luxury hotel.