The xxxHolic series was nice, but I felt it was hampered by the episodic format (I wrote a review for that too, if you're interested for some reason). The movie however, is absolutely fantastic. It was long enough to adequetely flesh out the story (the atmosphere in particular was awesome and creepy), and, as a movie, the production values were high enough so that the animation consistantly impressed me (the backgrounds are stunning and the characters move in a slightly gawky- though very smooth- way that I feel complements their character designs perfectly).
It isn't neccesary to have seen the series in order to enjoy the movie, but it might help. If anything, I would recommend watching the series first anyway, because this would make a really good finale.