I stayed in 105 Mercer, a historic brick building just steps from KEXP and Seattle Center. Overall I enjoyed the accommodations for what they were, but will probably not return.
Be warned that this is a true city apartment, the walls are thin, and the street noise is loud and persistent, and the parking situation is expensive and/or a nightmare. If you are able to overlook these issues–say you are visiting to enjoy downtown without a car, go out at night, or rehearse at the nearby artistic centers–then this is actually a great place to stay.