In the summer it get warm with temperatures above 30C during the day, but it's a dry heat and almost never stuffy. At night it cools down and the temperature is not a problem for sleeping.
During winter very rarely you'll see temperatures around the freezing point, it's more likely in the area of 8-18C.
Santiago has a mountain climate, which means that there are considerable fluctuations between night and day temperatures.
Weather-wise, Santiago is an excellent place to live, for me at least, never too warm, never too cold.