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.