Although a good description not only of physical attributes, but also of clothes, attitude, ambience and so on add a lot to the flavor of the scene, I have read extremely exciting stories when the actual spanking was almost non existant. I also have read very detailed spanking descriptions with pages of "whap" and "spank" and "swish" which (to me) sounded repetitive and ultimately boring.
I think that a good story can be appreciated regardless of the detail of description of the spanking and of the buttocks state prior and after the spanking. It all depends on author's skill and style.
