I stayed at the Parker Guest House on a business trip, wanting to avoid the usual business hotels downtown. And I wanted to live somewhere between Mission and Castro, as this was my old stomping ground when I lived in SF. And what a find: Parker House is fantastic; its very comfortable, set up with great care and a personal touch, and everybody is helpful, friendly and welcoming. The location couldn't be better: off Mission Dolores Park, so right between the Mission district and the Castro. Whether you are sightseeing or doing business in the financial district, almost everything is not more than a BART stop or two away.
On some websites, Parker advertises itself as a "gay hotel". As a straight man - traveling alone at that - this had the potential for some confusion. I didn't have to worry: everything was very relaxed, and I felt welcome and comfortable the entire time.
Bottom line: this is a fabulous place, and from now on my new go-to place when I have to go to SF. I will definitely be back !!!