[ziofill]

You are exactly right at the end when you say that particles are what makes a detector go click. Let me try to clarify further. The "particleness" is not about localization, but about energy quantization. In fact, a single photon can be very non-localized, because spatial position is a different Hilbert space and it can be entirely independent of the energy. So the packets you are referring to are particles only if in their energy Hilbert space they correspond to a well-defined photon numbers, and that's why the analogy with classical particles only goes so far. To add to the confusion, one can speak of packets localized in phase space (e.g. coherent states, like the light produced by a laser) and packets localized in physical space like a short pulse, and these refer to different Hilbert spaces.