A beautiful piece of work, superbly acted and directed. I really enjoy Gondry. But I found myself a little bored and distracted here. After you suspect and then realise the film's conceit, the director is still typically self-indulgent before bringing you to the end.
The Black Cauldron is quite scary for a kids' film. From my memories of the books, it's also a poor adaptation. It's often quite unclear what's happening and once everything breaks loose it's all wrapped up unexpectedly quickly.
As much as you were considering watching this… don't.