Great location in the heart of Union Square
My wife and I stayed at this wonderful hotel on Labor Day Weekend and we had such an amazing time. The location of the hotel is perfect, it is located near many restaurants, shopping, and nightlife. Transportation is also located everywhere in the form of the MUNI, BART, and cable streetcars. San Francisco is a very walkable city, many sights such as the Fisherman's Warf, Embarcadero Center, and North Beach are easily walkable from the J W Marriott, although if you mind walking long distances, I wouldn't recommend it.
There is a great breakfast place going up further on Post Street, which serves breakfast all day, a great place to head to after a night out. 
As for the hotel itself, the hotel was excellent. We stayed on the Club Level, giving us access to a special lounge which served breakfast, dinner, and drinks. The hotel has great amenities, outstanding fitness center, and the hotel is very clean and well-stocked with everything you need or may forget to bring with you. Additonally, the hotel is located right across the street from Ruby Skye, so for those of you who love nightclubs and nightlife, it doesn't get any better than Ruby Skye in San Francisco. 
I highly recommend staying at this property.