First off, this was a good book to read aloud. Chesterton's sometimes convoluted syntax can be straightened out nicely for pre-teens by shaping the phrases and emphasizing the right words, with a few tweaks to the diction here and there. In that sense, reading it to my kids was a bit like singing, an analogy I don't think GKC would mind. Even so, they were left a little foggy and confused by the end, but I think that may be appropriate.
The broad strokes that GKC paints with come off better when read aloud, I think. There's always something unrealistic about his stories, but this one works very well because it is deliberately framed as a nightmare of sorts.
I keep dithering on whether this would have been better at half its length or not. Reading through it a second time, I think the evenly paced nature of the successive "reveals" as the plot goes on feels unnecessary. But still, if the underlying point is to help the reader think differently about creation -- both as a noun and as a verb -- I think the book accomplished this admirably, while daring to assail the fortress of the biggest of the big questions, asking why bad things happen.
As always, it's difficult to know how to rate a classic, but I'll rate my reading experience more than the book itself. I was even gobsmacked a bit by a plaintive, simple question near the end and had to collect myself, so you can definitely say this book dug deep.