We stayed at Maria's Villa Kallisti-Volcano View apartment and everything about this place is amazing. The view alone is worth every penny. In order to stay here though, you need to physically be able to climb steep steps to and from the apartment. It takes about 3-5 minutes to go up and down (depending on how many small breaks you need to take) :)
Also - this place is located beneath one of the most popular places in Oia for pictures - which is pretty amazing. So for most of the day and especially at sunset, people are crowding the stairs and steps that you need to access to get to and from your place. Not a huge deal though but something you have to work through when staying there.
Overall, this apartment is VERY spacious and beautiful. The communication between Maria and ourselves was perfect and she arranged our pick up from the ferry port and our transfer to the airport. She provides a porter to help carry your bags to and from the apartment. I would definitely stay at Maria's places again!