We stayed a week in December and this place exceeded all our expectations. The unit is excellent - very clean and well maintained, very well suited for a large family or even a multi-family group. The kitchen is also well equipped.
Customer service was also excellent - although we were arriving on Christmas Day, the manager met us with the keys and showed us the property. During our stay the temperature dropped to -25, so we were pleasantly surprised that the unit is so warm.
We highly recommend both the property, and the time - winter in Yellowstone in wonderful. You will see more wildlife than you ever wished, because in winter bison, elk and even moose prefer plowed roads to deep snow, and pronghorns and bighorn sheep are grazing right next to the park entrance. And wolves and coyotes follow in their tracks. Getting caught in your car in the middle of bison herd is quite an experience, but in winter they are calmer and less feisty, so it is less dangerous than in summer.
There are well-groomed trails for cross-country skiing, and next door to the unit there is Parks' Fly shop, where you can rent your skis and snowshoes.They are cheaper and in much better condition than what you would find in the park. We highly recommend wildlife tours from Mammoth Lodge - these are a lot cheaper than snowmobile and snowcoach tours, and the guides are very knowledgeable.
We had a great time, and could not recommend the place highly enough.