A bad ending can completely spoil my enjoyment of a book. I like crime/thrillers as my commuting books, and in the last month I've read two books where the narrator turned out to be unreliable - and the perpetrator - in the last few pages. With very few if any indications beforehand. And in one case, not only was the narrator revealed to be unreliable, but the book finished without actually clearing up whether the narrator had committed the murder or not. I nearly threw my kindle across the room.

It's not that I don't like unreliable narrators or ambiguous endings, it's more when the ending is so completely unexpected and doesn't fit with the rest of the story.

We have the same rule. It's partly based on so many dreadful adaptations of books, partly as a way of encouraging slightly reluctant readers to try something different, and partly because I feel that if possible you should try the original before trying adaptations/versions/retellings.

My kids have adapted the rule to say "you have to experience the original first", so if the film or TV series came first, that's what you go for first.

Its not a hard and fast rule, I don't micromanage their reading, but my eldest was delighted when she accidentally followed the rule with Romeo and Juliet/West Side Story/These Violet Delights.