Counterpoint. I've visited the island about 15 times and only once over Xmas/New Years. Usually prefer low season, so you can see where this comment is heading.
St. Barths over Xmas was one of those once-in-a-lifetime experiences where once was enough for me. Yes, the celebs were even more in force than usual. Most restaurants added "festive" menus or items (caviar, truffles, DP at huge markups) or offered crazy expensive prix-fixe menus on holiday evenings. Parking a nightmare. Driving a nightmare. Noise. Crowds. Restaurants rarely seat an 8pm or 9pm reservation on time. More "beautiful people". It's a "scene", and if you like that sort of thing and your wallet can take it, then you'll probably be very happy (provided that you've planned reservations and rental cars well in advance as others have said).
If you're not looking for a "low-season" quiet/calm experience, bucket week is a good alternative to Xmas. The Island tends to be pretty full/busy but not like Xmas and of course you have all the beautiful sailboats, the race around the Island, etc.