It can be entertaining if done properly, for instance, an unexpected plot twist or a result of character development. However, when it is poorly written or the author uses it when it doesn't make sense, for example, only because it would look cool, then it can virtually destroy whatever enjoyment I might have had from the book.

A good example can be Codex Alera by Jim Butcher. It is a series of six books, where a cackling villain is intruduced in the first book. And then in the fifth book, he isn't a villain anymore, we are told that he was in fact a scheming good guy all along, and he no longer cackles that much. It was ridiculous. This, among other things, overweighs the good parts, and drags the series down into mediocrity.