A wonderful place in winter. It does get a bit nippy though so dress appropriately. Coldest day I had was minus 40 (C or F, same at -40).
Snow coach trips mean you have to abide by their schedule, unless you hire one yourself - about $900 a day I think!
If you go just before entry by car closes on the monday after the 1st w/end in November you can still drive in, which means you can get in place pre-dawn.
The weather can be varied at that time but often very cold and snowy - especially on the high elevations. Well worth doing. I may nip back for a week or 10/7 next year, closing date is Nov 5th, after which you can still get into Mammoth by car, and all along the north road.
Tim