This heartfelt rendition of "Have Yourself a Merry Little Christmas" by Matt Smith, Karen Gillan and Arthur Darvill is incredibly sweet—but I don't think anyone would mistake it for "good."
To be fair, in the softer sections, Gillan and Smith do sound pretty good. The way he and Gillan, who actually seems to have a rather decent voice, tear into this old holiday standard is full of the kind of joyful abandon that you'd want a Doctor to have. (And who knew Rory was so good tickling the ivories? No wonder Amy Pond loves him so much.)
Let's face it: No one seems to be having as good a time—doing anything—as Matt Smith.