We arrived just before sunset and the roof terrace was crowded with people who wanted to get the best shot: If I had been eating at a table facing the sunset at the time I would have been very annoyed with people leaning over me with their camera. Luckily, I was not, and 10 minutes later most bar guests had left and we got a table without reservations.
The food was excellent and we even got free dessert - I don't know if this was standard. The staff was very attentive and we got the best Mojitos on the island at the bar which had an impressive cocktail list.
Both restaurant and bar is placed on the roof top terrace from where you have a stunning view of the caldera, the sunset and the city of Oia. It's placed right on the main pedestrian street and therefore easy to find.