A must see for anyone who is a fan of Studio Ghibli (Nausicaa, Princess Mononoke, Spirited Away, Howl's Moving Castle, Totoro, Ponyo, et. al.) or The Borrowers. Newcomers to the genre of Anime may not appreciate it nearly as much though.
The problem is that it feels undeveloped probably in part because it's only the first book. Totoro was somewhat undeveloped too, but it was fully and satisfyingly magical. Arrietty must rely a little more on the lush hand-drawn animation though there are some touching and slightly tense scenes to help. I have not read The Borrowers (though my wife has and loved this movie), but I would have preferred a really long or multi-part movie covering all four(?) books.
Of course there is no magic or gods or demons or spirits in this movie, but the attention to detail is everything one would expect from Studio Ghibli. Besides the animation, there is the change in perspective in sound from the pov of the little people (louder and tinnier or more bass when they are confronted with human voices and generated noise) and also surface tension in liquids is more evident to them from rain, to standing water, to any liquid they consume. I thought surface tension would play some major role later on but it doesn't however it illustrates just how good the attention to detail is here.
The only reason to see this in the theater imho, is the non-CGI animation which is gorgeous. Should make a good rental. My 4 through 13 year olds (girl 4, boy 10, girl 13) were glued to the screen. My almost 17 year old girl didn't make it being busy with prom preparations. My almost 19 college son saw it on a date and didn't like it. Can't speak for my married daughters. Put this in with Totoro and Ponyo as a great movie for little kids.
I rate The Secret World of Arrietty 6 out of 10.
Btw, Studio Ghibli did an Earthsea film. It was uncharacteristically terrible imho. Started average, ended monstrously (in more ways than one). Just about every other film by them is at least pretty decent and imaginative, even Porco Rosso, though for some, it is an acquired taste.
"Surety you crave! Sauron gives none. If you sue for his clemency then you must do his bidding."
—Mouth of Sauron