My wife and I just spent a very enjoyable week at the Kouros. I can only repeat what other people have said about it: It's quiet, pleasant and clean, with friendly staff who genuinely try to help. We had an ordinary room on the first floor with a little balcony, perfectly adequate for our needs, with air-conditioning, a small safe for valuables, a fridge and a bathroom (hand-held shower, though). Most other guests were middle-aged or older, with a sprinkling of teenagers and only a few small children. They came from all over Europe and included some Americans. The pool is a reasonable size - the water is cold, but that was a definite plus in Santorini's hot sunshine, and there's about a dozen loungers and a small pool bar. The breakfasts (served on a sunny roof terrace) were fine, with something for everyone - rolls, bread, cereals, coffee or tea, fruit, cakes/pastries, cheeses, cold meats, bacon, sausages, eggs. The orange juice that some people haven't liked is actually orange squash mixed with water. The fact that the Kouros doesn't serve lunch or dinner is no disadvantage as there are lots of good restaurants and cafes within a few minutes' walk. The street on which the Kouros is located links the beach road and the main road and has some empty, overgrown plots and half-finished buildings on it, which could be off-putting, but it's quite safe even after dark (so long as you watch out for passing scooters and quad bikes!). I would happily go back there again.
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