One of the interpretations of quantum mechanics is the Many Worlds Interpretation which basically states that the universe as a whole develops like an unobserved quantum system, and any observation effects ("collapse of the wave function") are illusions which are caused by the observer getting entangled with the observed system, which effectively causes a split of his world into many worlds, one for each measurement outcome.
Most physicists are against this theory because those other worlds are, by definition, unobservable, and thus violate the common positivist view. However I'd be interested in what philosophers (especially non-positivist ones) think about that interpretation. Is it philosophically sound, or does it have fundamental problems?