For those interested in the proper QM calculation of what is observed in the classic S-G experiment, I warmly recommend this old paper (PDF), especially Appendix 1. It shows why it is possible to treat the wave function of a spin-half atom as a superposition of spin-up and spin-down trajectories, why the spin precession in magnetic field can be ignored, and how to account for the velocity variation and beam width in calculating the deflection distribution.