We stayed in Cusco for the month of December for a few big reasons:
Sure it was rainy season, tremendously cold at times, and homes typically are not heated here, so I am glad to be in warmer climates now. But the holidays here are wonderful, the food is pretty decent, and we met a lot of great people. And hiking some of the ruins is pretty amazing around here as well.
If you are thinking of going to Cusco, Dec through Feb is rainy season. Be prepared for cold, lots of rain, and difficulty on treks. In other words, think about going a different time of year.