This is a science fiction oldie, and one that is usually dealt with rather poorly. (Giving humans conscious control over fundamental physics is all too often used as a deus ex machina to solve the plot problems at the end of a novel: Orson Scott Card's Xenocide is a recent example.) Egan makes one big (massively implausible?) assumption — that wave function collapse is the responsibility of a particular part of the brain and that with the right neural modification people can learn to avoid doing it, producing a "smeared out" universe — but otherwise his scenario is internally consistent. Even more importantly, Quarantine actually tries to "follow through" on the consequences of its assumptions, and manages to bring something of their full metaphysical immensity home to the reader. If you are interested in this kind of philosophical exploration of quantum mechanics then Quarantine will fascinate; if not you will probably find it rather frustrating.