4 stars but 5 if you're watching in japanese with overdubs. The "inspired choice" of using big-name hollywood actors for voices, a rising tradition in big-budget animations, fails here. In Japan, voice-acting is a respected and coveted discipline, and the artistic integrity of it is important. In Japanese, the voices were appropriate and made me laugh, or cry, at moments, purely on delivery alone. In english, the voice acting was 'ok' at best, and distracting (Liam Neeson) at worst.
Visually this is a stunning film, and the plot, while it loses something in it's third quarter, is at least original, despite it's origins.
You can't really trump a Miyazaki film for explorative imaginative scenes, and the best of these are during the water-boating/flood sequences, with a plethora of extinct sealife exposed.
Despite a weak end and lack of an antagonist or any real danger, a solid recommend from me. Just watch in Japanese if you can deal with subtitles.