If someone tells you there is a twist at the end of a story, do you spend the whole book thinking about what the twist might be? If so, does that stop you really being present to enjoy the story, or does the anticipation actually add to your enjoyment?
Most people read books that have a twist or a mystery specifically because they enjoy that type of story. They spend time trying to work out what the twist will be.
Unfortunately when you read a book or see a movie with a twist and that twist is obvious or if the author or film maker has not provided clues to the reader or viewer, then they will feel cheated by the twist.
I have only ever read one book with a big twist ending that I didn't know would have a twist ending. I almost gave up on the book halfway through because it without the twist the book was stupid while the twist made the book absolutely brilliant.
Outside of books, I love twists in short stories, because you go from anticipation to satisfaction in a simple sitting. If the story fails then I don't feel disappointed.
I don't think saying there is twist at the end is a spoiler. I think that would be part of a good review that encourages someone to read it, and be excited about finding out what happens. It is only a spoiler if they actually reveal the ending.
Saying that, I probably would spend a lot of time trying to figure out what the twist might be, and that might make me lose concentration about what is happening at the bit I am reading.
As long as these people refrain from giving out the twist, it's okay with me. I just can't bear when people can't contain their excitement and blurt out the twist or ask you to guess the most interesting part and start giggling and giving glaring hints about the climax.. that ruins my essence of surprise/shock!
With some books I find I enjoy them a lot more if I started reading knowing nothing at all about the book. I sometimes read a book knowing that there's probably going to be a twist, and I can enjoy those anyway if it's a good one, but if I spend too much time thinking about what the twist will be that gets distracting. Also, occasionally the twist isn't as good as what I had guessed would happen.
I consider it rude, but not exactly a spoiler. If I am reading a book, I want to experience the story for myself, I don't want to be told what to expect because that taints my experience. So I wouldn't consider it a spoiler, although it can spoil the experience.