Since you posted this on the Au Naturel side, I will confine my reply to that aspect...
We were there the first week of January last year (so not right at Christmas). I will NEVER go back to CTI anytime around there, and maybe never. Why? Basically a full half of the week, three and one half days, were Red Flag days and the island (and watersports) were closed because of the wind. The biggest things for me when I go on vacation are the Au Naturel aspect and sailing. We were able to go over to CSS for two of the Red Flag days, so that helped lessen the impact, but basically losing out on two days of my vacation sucked.
CTI was nice (although not my favorite resort by a long shot) but the wind was a "show stopper" for me. I would gladly go back to CSS and our next trip will be to CN, but I doubt I will ever see CTI again except as a trading places day from CSS. I did really like the island but the uncertainty ruined it for me.
We spent Christmas at CTI probably 4 or 5 years ago now...we had an awful experience with lost luggage, 4-5 days of straight rain and at the end of the trip my wife got salmonella. That said, we've been back twice since then and are itching for our next child to be born so we can head back (to CSS though) again.
Christmas at CTI was actually the first nice weather day that the resort had so people were pretty much going bananas all day with watersports, the island, etc. I don't want to ruin the surprise with how Santa travels to the resort but it was really fun and since it's an all adult resort, the adults were like kids and everyone sat on Santa's lap for pictures and presents (we got a small bottle of Appleton Rum) and the staff was dressed as elves and there was Christmas music blaring and I don't think anyone was missing being at home for Christmas. That was our first and only Christmas away from family and now that we have kids we probably wouldn't do it again (at least until they're a lot older) but it was a really fun time. We've stayed at CTI just that one time and we've been to CSS twice and we really like CSS for the AN experience although CTI was great for a first time experience. If you've got anymore questions, just reply to this message...I don't check it that often because it makes me miss "home" too much.