I have two Vista 64 systems with very comparable CPUs, motherboards, memory, etc, each of which initially used the 6400AAKS as a boot drive, and later, velociraptors (one with a 150GB, and the other with a 300GB version). The most noticeable improvement with the VRs is reduced application loading time, and performance when simultaneously accessing different programs and files across the same drive. Opening programs that have not been cached to RAM is pretty much instantaneous with the velociraptors, compared to the 6400AAKS.
I agree that they are a luxury; still, the enhanced responsiveness of the system was of sufficient significance to justify the cost (for me).