Maria's place is beautiful, overlooking the caldera. The check-in process is excellent. Her colleague, Ef, will take care of everything for you. Do note that you will need to climb about 100 stairs each way from the apartment, but Ef's team of porters will carry your luggage up and down those stairs for check-in and check-out.
I highly recommend booking your transfer from the airport/ferry with Ef, as transportation can be tough to Oia, requiring a bus transfer or expensive taxi.