A Very Harold & Kumar Christmas is a very Harold & Kumar movie, it's just not a very good movie. It is, in fact, a very stupid movie. The story and the jokes feel like rehashed sloppy second versions of the first one (and probably the second one too, but I never saw it). It is just as rude, crass, and vulgar as every other stoner comedy made in the last 10 years. And it is just as funny as well--which is to say only occasionally funny. The jokes, when they do hit home, can be great. But they can be abysmal as well. The shining light in this series is Neil Patrick Harris, and he does not disappoint here. Unfortunately, he is not enough to make this a memorable movie in any sense of the word. The best I can say about Harold & Kumar is that it is mindless entertainment, with a big focus on mindless. If I were Kal Penn, I probably wouldn't have quit my day job to make this movie.
IMDb link: http://www.imdb.com/title/tt1268799/