Essential in a good mystery is the beloved "twist." The sudden unexpected turn of events or realization that things are not what they had seemed to be up to this point. To me ya gotta have 'em, and the more the merrier. Sure you can overdo it, but several twists in a story keep you guessing, make you wonder, and add that surprise! surprise! element to a gripping story.
In a classic whodunit, a good author can have you feeling for certain that "this person" is guilty, then for sure it's "that person," but oh, no! - you're kidding me - it was her all along? And then still you weren't correct. It was him that did it!
Love it. Plot twists. They add to stories in many genres, but the mystery/suspense genre, to me, really shines when this ingredient is well done. Again, not overdone. While not a novel, the last "Mission Impossible" movie (I think it was the last one) with Tom Cruise went stupid with the mask wearing thing. I thought I was going to puke if just one more person tore off their mask right at the height of a pivotal scene to reveal that they were impersonating someone, whether good guy or bad guy. That's weak redundant overuse of the same old same old.
My favorite twist is when the protagonist, all book long, turns out to be the perpetrator.
What's your take on plot twists? How much is good, how many is too many, and what kind is your favorite?