Staying at the Magma Resort right now. To summarize, it's a very nice property if you don't mind the location, but they play games with suite upgrades.
We booked last minute, four nights at 35,000 points per night for a standard room. My wife is a Globalist and we used one award certificate to upgrade with a suite. The Hyatt agent told us the upgrade was confirmed, but when we received the confirmation email we noticed the room rate had been changed from points to cash at more than 1,000 euros per night. We emailed our Hyatt concierge, who changed it back to points, but added that the hotel had "inventory management" problems and did not have suites available.
At check in we were upgraded to a room with a private pool but told no suites were available. Meanwhile the hotel is still showing standard suites for sale for our dates so I don't think they are being honest. Didn't want to make a scene so left it at that.
The property feels brand new and well built. By European standards, room size is decent, though we don't have much use for the private pool. The main pool is quite nice, and feels bigger than what I expected based on the photos. It's narrow and shallow, but quite long; you can sort of do laps if you want. Supposedly the hotel is full, but doesn't feel crowded. Breakfast was quite good. Obviously no caldera views from this location, but you can see the sea and the island of Anafi on the horizon.
If you don't have a car, the location may or may not work for you. The hotel runs four free shuttles a day to Fira and to the beach. They'll also do unscheduled shuttles at a steep price (40 euros from the airport, 25 euros to get us back to the hotel from Fira after dinner). If you don't mind walking, it's about 10 minutes to the village of Vourvoulos and 30 minutes to the caldera. Hot, not particularly scenic, and a bit dodgy at night (some stretches on busy roads), but doable IMO.
If you're happy to chill by the pool for a couple of days, get your caldera views elsewhere, and/or plan around the limited transportation options, it's a nice hotel. It's a very busy and expensive summer for travel over most of Europe, so it's good to be in a place that has nice rooms and doesn't feel crowded. Overall I'm happy we came, but I might have booked elsewhere if I knew they would play games with their suites. If you value 35,000 points around $500, there are lots of options at this price point.