We spent the week of New Year's Eve in the Grandes Rousses Hotel. It was a real 5-star-hotel experience. The staff was friendly and helpful, the lobby/bar area/restaurant was modern and cosy. The fact that the ski rental was in the hotel building itself was very comfortable.
Our room (a deluxe room for 1 adult and 2 teenagers) in the new building was as stylish as the bar. The beds (even the single bed) were really comfy with the best duvets I have ever experienced in France.
The only reason for not giving 5 stars is the price of the hotel: 30 EUR per day for the breakfast. 30 EUR per day for parking on the street in front of the hotel. 25 EUR for 1 hour for the spa. 10 EUR for a tea in the bar. Plus an expensive room rate.
I am not sure if we had enjoyed the "older" rooms in the main building as much: Their view on the mountains is better, however, the style of this building appears a bit outdated.