(Warning: Some moderate spoilers)
The last eighty or so pages of Howl's Moving Castle were incredible. A dozen different things were finally revealed, relationships changed, realizations struck. Even if the first 80% was only "good", I must say that the book as a whole is "excellent". It gets a 93% in my book.
The character arcs were made spectacular by the nature of the tale's narrator. He never gave a nod at the changes, and in some cases left no hints whatsoever, so the arcs had to be discerned and implied from reading. While Sophie and Howl's relationship arc jumped more than it probably should have at the end, it was almost inevitable in hindsight. I suspected it more and more as the book went on, which eludes to hidden pieces of arc that I picked up on. The MG elements and air of the novel kept things from getting too dense to pick up on.
Howl's Moving Castle has a splendid setting. It's basically our world in the near past plus magic. Everything feels more serene than reality, and truly it is. It's one tiny step toward utopian, just enough for a "feel good" atmosphere.
The plot felt quite episodic in places, which it sort of was in a few cases, but everything tied together well enough. Various ingredients went into the pot slowly, then came to a simmer at the three-quarters mark, to end at a full boil. It made for some nice stew.
If you like MG fantasy, I strongly recommend you pick up Howl's Moving Castle. Keep in mind, there are two sequels. I wish I had known that earlier...