IGN (Score: 7.5)
"Costume Quest does a lot of things right – it’s humorous, has a great art style that fits the childish theme, a decent battle system, and costumes that double as super powers... But despite these great qualities, the game does have its shortcomings..."
"Costume Quest isn't the kind of sugary indulgence that's likely to make you feel sick. Rather, it's a light and delicious treat that's perfect for enjoying between this winter's gaming meals without the risk of ruining your appetite."
Gamespot (Score: 7.0)
"Without question, the art and overall premise serve as Costume Quest's biggest hook, and these things are ultimately what drive the game and turn it into something much more than a superficial RPG. Still, whether or not you can overlook and overcome its initial shortcomings is tied directly to how much you buy into its premise on a nostalgic and comedic level."
"Well, Double Fine's done it: They've made a game focused on the concept of trick-or-treating and managed to land on the right side of "gimmick." Costume Quest is, plain and simple, a fantastic embodiment of childhood imagination mixed with a healthy heap of cockiness."
Source: IGN
SurplusGamer
I'm not a huge fan of IGN either, but let's be fans, not fanboys.
Did I come across as being a fanboy? I just think that, traditionally, IGN have been unfair in reviewing downloadable games. Their relatively average score doesn't surprise me.
Having played some of it, the issues are nothing spectacularly worrying, but depending on how much an individual reviewer cares about them or is willing to look past them, I can see why peoples' mileage would vary. Still, apart from the 6/10 review which frankly read more like a 4 in the review text, these are all respectable scores.