Great experience at the Cornell
My wife and I rented a car from National Car Rental at San Francisco International airport when we arrived in SF. We immediately headed south on the coast (Half Moon Bay and Monterey) for a few days before returning to the city for the rest of the trip. We returned the car at the National/Alamo location right across the street from the Cornell. This was convenient, because you don't really need a car in SF (especially if you are staying in Union Square) and parking is expensive. 
We stayed two nights at the Cornell. The staff were all friendly and helpful and the hotel was clean and, as others have said, full of character. The cage elevator is obviously original to the building and very cool. Each floor of the hotel is dedicated to a different artist and the hallways are decorated with prints of their work. We stayed on the "Gaugin floor" in a room at the front of the hotel with windows facing Bush street. We could hear street noises (especially with the windows open) but didn't hear any noise from the hallways or other rooms. The TV in our room was old school (not a flat screen), but that wasn't important to us and we never turned it on. The free, wireless Internet was fast. The bathroom was small but clean and had a great claw foot tub with a shower that worked well (good water pressure, nice and hot). 
The Jeanne D'Arc restaurant located in the basement was great. The free breakfasts were great and service was fast. We ate dinner there one of the nights and everything was delicious. Our server, Dan, gave great advice on wine. 
Overall, it was a great experience at the Cornell. We would definitely recommend it and would stay there again. I wish we had spent more nights there because we moved on to a bigger hotel and paid nearly twice the price for a generic experience with none of the Cornell's charm.