Marc's spot is a great and affordable choice for solo travelers or couples who need a comfortable (albeit no-frills), private studio in one of the best parts of San Francisco. The apartment was very comfortable and super convenient to get to and check in at, the wifi was fast and the neighborhood is great.
The only warnings I have about it are: First, it really is just a bathroom and a bedroom (so no kitchen, don't expect to cook anything). Second, the bedroom faces the street on the first floor, so it's not a good choice if you're noise-sensitive.