We spent one week in Kostas apartment in Kastraki and I wish we would have had more time! It is just perfectly located at a beautiful beach, this time of the year pretty empty but perfect conditions for swimming and to spent the day at the beach.
Kostas is a great host, replied instantly and was very helpful organizing some additional stuff, so if you have any issue just text him!
The apartment is in great shape, nicely furnished (even better as shown on the photos), has everything you'll need and is super clean. The terrace has a stunning view over the ocean and I'm not going to start describing the sunset - just perfect! There is a sun shed so you can sit there at any time of the day.
You can take the local bus - it's about 25 minutes from Chora (Naxos Town), but I would recommend to rent a car for some days to visit some villages on the island (15 to 30 min from the apartment), go for a hike at Mount Zas (!) or to the western shore (30 min).
Grocery store is a 2 min walk.
Anything you could wish for!