I have learned quantum mechanics as a young man, first from the book by Dirac, and then form a multitude of other excellent textbooks. The theory appeared bizarre and marvelous, but it made perfectly sense to me. The world, as Shakespeare put it, is "strange and admirable", but it is coherent. I could not understand why people remained unhappy with such a clear and rational theory. In particular, I could not understand why some people lost their time on a non-problem called the "interpretation of quantum mechanics".
I have remained of this opinion for many years. Then I moved to Pittsburgh, to work in the group of Ted Newman, great relativist and one of the most brilliant minds in the generation before mine. While there, the experiments made by the team of Alain Aspect Aspect at Orsay, in France, which confirmed spectacularly some of the strangest predictions of quantum mechanics, prompted a long period of discussion in our group. Basically, Ted claimed that quantum theory made no sense. I claimed that it does perfectly, since it is able to predict unambiguously the probability distribution of any conceivable observation.
Long time has passed, and I have changed my mind. Ted's arguments have finally convinced me: I was wrong, and he was right. I have slowly came to realize that in its most common textbook version, quantum mechanics makes sense as a theory of a small portion of the universe, a "system", only under the assumption that something else in the universe fails to obey quantum mechanics. Hence it becomes self contradictory, in its usual version, if we take it as a general description of all physical systems of the universe. Or, at least, there is still something key to understand, with respect to it.
This change of opinion has motivated me to start of a novel line of investigation, which I have called "relational quantum mechanics". It has also affected substantially my work in quantum gravity, taking me to consider a different sort of observable quantities as natural probes of quantum spacetime.
I am now sure that quantum theory has still much to tell us about the deep structure of the world. Unless I'll change my mind again, of course.