In order to fully appreciate the content of Weinberg's  book, one needs  to study Paul Dirac's Quantum Mechanics or Albert Messiah's Quantum Mechanics before hand. Beginners will have difficulty appreciating the physics behind the mathematical rigor (ie. abstractness). Weinberg took the same approach as Dirac in discussing the complexity of quantum mechanics. However Weinberg discussed additional topics not found in standard quantum mechanics books; such as different  interpretation of the quantum world, Open systems using density matrix, quantum measurements, quantum entanglement, Berry phase, Rabi oscillations, Feynman's path integral formulation, gauge invariance, quantum computation. I gained a better  understanding of quantum physics from Weinberg's exposition during the course of my research on the foundation of quantum mechanics.