It's a soft four, and I feel I'm being generous. Possible that expectations were raised unreasonably by its fame.
Its main strength, perhaps, is in the concept. Treating cats as people, with their varying personalities and exploits. The variety of characters is entertaining.
As individual poems, however, the collection impresses less. Two stand-outs are "Macavity: The Mystery Cat" and "Gus: The Theater Cat". These are also two of the most popular songs from the musical based on these poems. (Sarah Brightman did an oddly sexy version of "Macavity", but I digress.)