Like Jennifer said, only if it is well done. I think Christmas episodes are best suited to comedies. There are some excellent examples, such as The Vicar of Dibley, Dinnerladies, and Miranda. But there are some awful ones too, like My Family.
Not particularly. The Bold and the Beautiful does it often, and I can appreciate that they want to connect with their audience, but I don't like it when they directly address the camera and wish everybody a merry Christmas. It ruins the fantasy of it all.