Based on my experience (2005 onwards) the snow should be okay. The Espace Killy usually opens in the first weekend of December (or last of November) so you can expect most sectors to be joined up and the all important Tignes-Val D'Isere links to be operational. It's touch and go whether all the runs to Les Brev will be open and the Chaudannes and Paquis lifts may not be running yet, but they could be - they're always open for Christmas and the area is heavily snow-machined.
They haven't always been able to run Le Critérium de la première neige de Val d'Isère (11th-13th, 18th-20th Dec this year I think) the last few years because they need something like a metre's depth at the bottom of the OK/Orange piste, but for punters (well, for me) the runs are good enough even if occasionally you get the odd stone kicked up, or tuft of grass after some strong wind.
In conclusion I'd say I've had some really enjoyable skiing at that time of year blowing away the cobwebs, hardly anyone else around.