While not cheap by any means, the Top of the Caldera is worth it completely for the location (perfectly situated at the top and middle of Oia, walking distance to everything, close to public parking), and jacuzzi/outdoor space (there is absolutely nothing better than waking up to that view!). My husband and I felt like we had died and gone to heaven watching the view from the jacuzzi - Santorini is a truly special place and we felt this was worth the splurge itself. In addition, despite the ideal location near shops/restaurants, the bedroom was also 100% quiet, which was great as I am a light sleeper.
That said, the "soft" product could be improved, as communication with Fanis&Tina was extremely slow and I needed to chase them multiple times to get answers to basic questions before we arrived. The only time they were quick to reach out to me was to confirm what time we were checking out! As others have mentioned, this is truly a business for them (which is great and they are obviously successful at it!) but the property lacked some of the creature comforts that we would expect at these prices, such as shampoo/conditioner or even fresh coffee.
Overall, a highly recommended property for those who know what they want and do not require a lot of hand-holding while preparing for their trip!