I thought Animal Treasure Island was a fun, but EXTREMELY light and insubstantial movie. It felt a lot like a Miyazaki movie in tone, but without much character development and absolutely no subtext or interesting themes. It was completely devoid of anything meaningful that would stick with you beyond the ending credits.
As I said though, it was really enjoyable to watch. And sometimes in Miyazaki's movies, I feel like the 'message' sort of bogs down the entertainment aspect anyway, so it's a nice counterpoint to his movies. It sort of feels like the first half of Castle in the Sky (which is very fun), in that you're watching a couple of kids fighting and getting chased by pirates. I'd recommend it, but only if you don't expect anything big.
EDIT:
Upon reading the review, it seems I feel the exact same way about it as Radical Raven.