I agree with Susan in terms of the grinding and pacing issues. However, I completely disagree with her negative review of the characters, particularly Oliver. She claims Oliver is:
"...a brainless gorp of a child who lacks the deductive reasoning skill of your average sandwich bag."
This, in my opinion, shows that she has misunderstood what the character is about. Oliver is meant to show a lack of reasoning because a) he is a child and is naive to many things in the world and b) because he purposefully disregards reasoning in favour of emotion. The latter of these is exemplified by the fact that Oliver is so trusting of a doll which came to life (Mr. Drippy) that he will follow it into a parallel world and will try to save that world from its evil ruler, just for a chance to save his Mum. All the while, he rarely asks any questions nor seems phased by the extreme weirdness of it all or the possible consequences (how many of us could say the same in his situation?). He just wants his Mum back and will do anything to get her. This is what makes him exceptional. His age also seems to play a significant role because kids are, generally, more fearless and, when faced with a challenging prospect (such as diving off the top diving board at a local swimming bath), they often focus on the rewards of the task (getting friends' approval)rather than the possible consequences (potential injuries). As we get older, and particularly when we have kids, our sensibilities come out and we allow potential consequences (such as injuries) stop us from doing certain things. Therefore, I believe that the writers have created a refreshingly innocent and selfless character, whose lack of reasoning skills, or general care for reasoning, make him an even greater hero. Better than the ever-increasing gun toting male, who prides himself on the enjoyment he gets out of killing and being selfish, anyway.
I might well have read too much into the innocent nature of the character, but, given the meticulous nature of Studio Ghibli and Level-5, it seems reasonable to assume that there was significance in making Oliver a naive and ignorant pre teen in the context of the story. It's a common theme with many Studio Ghibli films, and is partly what makes the extraordinary events all the more compelling.