I've been there once and I can't wait to go back. Santorini is the perfect combination of paradisiacal and rustic, of glamour and genuineness, with its white and blue buildings, colourful streets filled with handicraft stores, delicious restaurants and, of course, a gorgeous torquoise sea. Everywhere you look there are flowers and pools, wood and stone, in a contrast of colours and textures you can never forget. The sunset on the vulcano side is one of the most awesome things you will ever see and at night, when the lights turn on and the sky turns dark, the view over the caldera side of the island is breathtaking.
Long story short... When you get there, you just don't want to leave anymore.