We had a great time staying in Ultimo! There are so many cool cafes and shops nearby, including the Broadway mall. We walked to the Powerhouse museum (so close!) as well as into Chinatown, Darling Harbor and the downtown.
Let the host know if you are planning to use the sofa bed so the linen is there when you get in! The apartment is next to a busy road: I needed to sleep with my earplugs from the plane, but the neighborhood is otherwise not noisy.