Although I initially visited Pirin National Park WHS in summer 2010, I went back there for skiing at the Bansko ski resort, located within the Pirin NP, in January.
Both times I took the narrow gorge train from Septemvri to Bansko and took bus back to Sofia. The scenery from the train was quite pleasant both in summer and in winter.
As it turns out, the ski resort is located within the buffer zone of the WHS; however, if you look at the map of the WHS, you see that the buffer zone is almost surrounded by the core zone, which means that it appears as if the ski resort is oddly protected by the core zone of the WHS.
This photo shows part of the pistes in the buffer zone to the left and the core zone to the right.