I did a 2 night stay there this month for a conference down the street.
Booked a standard King and pre-arrival upgraded as a Diamond to a Skyview Corner Room on the 28th floor with superb views. (The hotel has 32 floors.) ^
On check-in asked some Hilton specific questions and it seems at least to me, the front desk is clued in about the major parts of the HHonors program. Uniformly friendly staff and you had the feeling the reflag to Hilton was a good thing to them. Asked if the room had a mini fridge and they proposed sending one up no charge, which I said yes.
The public areas are very contemporary, very new and somewhat atypical for a standard Hilton one sees in city downtowns.
The location is quite good, but keep in mind, the Tenderloin district is not far away and you need to know where are going. Two SF police are detailed from 6 pm there mostly to keep the rifraff and homeless away. I never felt unsafe, but then I am SF often so know where to go in the evening. Across the street is the the Westlake Center, the biggest downtown mall. Plenty of restaurant choices and BART is right outside the front door.
As for the room I had I would estimate it at about 400 Sq ft and never felt cramped. Overall condition: pretty good to excellent. Everything worked, save for it was a bit slow one morning getting hot water in the shower. Wifi was spotty one evening and then fine the rest. Wifi in public areas was usable.
Diamonds are given a coupon for full hot buffet, which was fine for me, including Asian options, as well as salmon and eggs benedict. You can apply this to an ala carte choice and gratuity is included with the coupon. Breakfast service was certainly above average in all cases.
I had dinner in the lounge one evening and the standard kinds of offerings were fine. Decent wine list too. Not spectacular, not awful.
In our corporate booking system it is rated 4*, over the other downtown Hiltons which are rated 3* and I would agree. I have stayed at the other two downtown and found this to be the best option for quality overall.
http://www.parc55hotel.com/ Has some good pictures which really give you the flavor of the hotel.
Nits:
Parking is obscenely expensive if you are from outside SF $58.00 a day - valet only.
The exterior windows could have been cleaner given they advertise the views.
Overall: Solid downtown 4* star hotel.