First, let me say, this is the best Christmas movie I've seen in a long time. While the idea behind it seems to be a stereotypical Hallmark Christmas movie, the movie addresses the issues in a decidedly non- Hallmark way. It doesn't just resolve with everyone forgiving each other because of "Christmas Magic" and it doesn't treat serious issues in a heavy handed way.
While it has plenty of comedic moments, you shouldn't consider it a comedy. If you do, then it fails because, while the antics of the characters are pretty funny, it isn't a fall-off-your-seat-laughing sort of movie. It is a genuinely good movie about a dysfunctional family, children questioning the magic of the world, and a father who is desperately trying to be a better dad than his was.
It is extremely well written and the performances by Joel McHale, Lauren Graham, Candice Bergen, and (of course) Robin Williams make this movie a must see for any Christmas or Robin Williams fans.