This past trip was my third trip in as many years. The snow is very dry and deep and consistent. Skiing at Christmas (early season) is a bit of a gamble but I would put my money on Niseko over any other place in the world to have good snow at that time of year. This year was especially awesome, with already about a 2 meter base in the village, and fresh snow almost every day.
The drawback of Niseko is that there aren't many steeps. Skiing "packed powder" for a week at Niseko would eventually get boring, compared to a bigger resort like Whistler. But Niseko is truely a "powder resort". It's sick. The snow comes from squals from Siberian winds passing over the Sea of Japan. Unlike low pressure system storms, which tend to dump for a day or two, Niseko often gets 30cm+ dumps day after day after day, and the snow cold and dry.
Most North Americans wouldn't come all the way to Japan to ski because they think they have the best snow on earth, but they're wrong.