We enjoyed our stay in Oia. Marcos Rooms was exactly as described. The 3 bed room had actually 1 double bed and 2 single beds which was plenty of room for 2! The rooms are perfectly situated, close to everything while very calm and quiet. Breakfast and drinks on the roof top were much appreciated...
The only thing that could be improved is the bathroom: you can't take a shower without getting everything wet (toilet, sink and shower hose in a tiny space). But then again, on an island you're not supposed to take long showers anyway...
For that price, we definitely recommend Marcos Rooms.