What makes a book adaptation successful: fidelity or spirit?

[NathanD]

I just finished watching the new series adaptation of a beloved novel, and I'm left feeling conflicted about the changes made to the plot and characters. It's made me think about what makes a book adaptation successful in my eyes. For other readers and viewers, what do you consider the most important element for a good adaptation—is it strict fidelity to the source material, capturing the spirit and themes, or creating a compelling standalone piece of cinema? I'm interested in examples of adaptations that significantly deviated from the book but you felt were brilliant, and conversely, ones that were slavishly faithful yet fell flat. How do you separate your experience of the book from your judgment of the film or series?