Bitmob: Sweet is the operative word with Costume Quest. Not only does candy feature heavily in the story (heck, it's even the currency), but the game is so disarmingly sweet that I feel bad critiquing it.
And Costume Quest is not an easy game to critique. How you'll feel about it depends on how you approach it. Are you a parent interested in a gateway game to get your kid interested in RPGs? Are you yourself a kid at heart? Then, you can't do much better than Costume Quest. But if you're an RPG fanatic looking for a meaty experience, you'll likely be disappointed.