Great location, average JW Marriott.
In recent years I stay at the JW when I travel to San Francisco. The location--just off Union Square--is great. Walks to downtown, BART, Lombard Street, the Caltrain stain, and other parts of the city are very easy to do.
As always, I was greeted by extremely courteous doorman and front desk personnel. The clerk who checked me in immediately recognized my Marriott Gold Elite status. Unfortunately, a suite was not available so I was upgraded to a corner room. It was okay (clean, etc.), but nothing to write home about. The concierge lounge was okay but, again, nothing to write home about. 
I always eat breakfast when I stay at this property. The buffet is average. The offerings are not that plentiful, perhaps because the buffet is crammed into a corner. Nevertheless, the cost with tip is in the $25-30 range. The waitstaff is always very pleasant.
A big downside to the hotel is the expense. I'm sure the location is at least partially responsible. But for some reason I've never felt that what the hotel provides in terms of amenities, elegance (or lack thereof) justifies the costs. But despite that, it still remains my "go to" hotel when I visit San Francisco.