I was looking forward to this when I heard of it as I've really grown to like the series. I haven't played the games, but the series is quite good, and this special is no exception. It continues the story from where the first season left off, and it is cool to see what the characters are up to after defeating Bob. Also, the Christmas setting is a nice change of pace from the fall scenery of the first season.
Overall, this is a very entertaining special with the same level of quality present in the first season. I love that it managed to create it's own story without resorting to any of the tired Christmas special cliches like having the characters help out Santa Claus for example. It's cool to see how the kids, Norm, Oona, and the Grubins are all a team now, and how all the character development that we saw in Season 1 continues here. I wont spoil anything in this review, but it is cool to see how the characters are starting to trust each other more, and there is a nice moment in the climax that showcases how the Grubins have truly changed for the better. There is some cool action as always, but most of it is towards the end here, and there isn't too much of it throughout the special. However, there is enough in this to keep you interested, and the story is entertaining with all of the character interactions and humor. I am also glad the show continues to not devolve into sophomoric levels with awful toilet humor.
Any nitpicks I would say are minor, and don't take away from the overall special. The new antagonist is not that interesting, and she can get annoying in some scenes, but overall, she doesn't bring it down at all. It's also sad we don't get to see the kids interact with their parents in this, but I know you can't squeeze in every character even in a special that's over 30 minutes. Norm also takes somewhat of a backseat, and doesn't appear that much in the middle of the story. However, the positives easily outweigh any nitpicks, and if you like the show then this is certainly worth watching.
I believe this to be a well done special, and even though it's not necessarily among my favorite Christmas cartoons of all time, I still think it's one of the better Christmas cartoons I've seen in recent years, and possibly one of the best I've seen since Phineas and Ferb's Christmas Vacation. I really hope Amazon chooses to continue Costume Quest, and that it doesn't end up dying after only one or two seasons. The series is very well done, and I would love to see more of it.