How is it early season (December)?
It really depends, but it is usually good. Last year was an absurd aberration, so don't expect that again. The mountain should be mostly covered by then, and even if you don't get fresh snow you should be able to get classic Mammoth windbuff in the noids, climax, hangers, MJB's, drop/wipe, and the avy chutes. Actually, call 'maybe' on hangers and MJB's, and throw the head chutes in there as well. Philippe's probably won't be open yet, and the top of the world will probably not be doable, but you never know.
Here is the good thing: The Mammoth wind machine tends to keep things pretty smooth, so you can often have great windbuff in the steeps even if it has not snowed in a couple of weeks. Just watch the wind patterns, and try to figure out where it is depositing the snow. Getting to ski the Paranoids in nice windbuff is pretty classic, and you have a very good shot at getting that type of skiing.
Could it suck? Yes, but probably not. It's too bad you can't come in February, as the base will be better, but it should be good in December.
What is the snow like?
It is not as light as Utah pow, but it is still nice. It is lighter than Tahoe snow, and the coastal stuff sticks to the steep rocky areas pretty well. It does get some pretty light stuff sometimes too. My favorite storms are the ones that start out wet and cool gradually.
How often and how much does it snow?
Often and much. Mammoth gets something like 400 inches a year, and it is prone to massive storms from time to time. 5 foot dumps are not all that uncommon, and I've seen much bigger ones come through. The storms will close the upper mountain, so it's not a place like Snowbird where the gondola sometimes runs in storms. The lower mountain is great on storm days though.
How are the crowds?
It will be crowded when you are there. Having said that, Mammoth is very good at spreading crowds. It never have too big of an issue there, even when it is packed. The longest lines I wait in are maybe 10 minutes. They can be longer, but that is pretty rare.
Any reason not to go there?
As alluded too by other posters, the price. If I did not have a season pass it would be REALLY expensive. I also don't know how you are getting there. Will you fly in directly? Mammoth does not get much non-socal traffic because it is hard to get to from anywhere else. Socal still makes the mountain crowded at times, but not many people from the east choose it over Vail. They should, as it blows Vail's doors off in terms of terrain, but I'm glad they don't. I should probably not be talking it up.
Whenever you plan a trip in advance, you are always taking a chance. I am a yuppie, not a local, and I ski Mammoth 2-3 weekends a month and Mt. Baldy when I don't make the 5.5 hour drive. Because I have to get a ski lease every year, so I have a place to stay all season, my ski budget does not allow for many trips. I am always scared when I book a trip, as I could go to Utah, have it suck, and have Mammoth be great while I'm gone. If you live in the east, you don't have to worry too much, but if you live somewhere cool, like Seattle, you might miss out. That's just a risk you take with every trip though.
I surf too, and part of the reason I like Costa Rica is that it is very consistent. It always has waves, so I know that even if socal is going off I will still score in CR. Mammoth is a bit like that, if you go when there is a base, which there should be when you are going, you should get good windbuff turns no matter when the last snow was. It is a very good risk, IMO.
