I bought the WTPro/ST recently and have been putting it through its paces. I bought it for the ICD, which is like a Spice implementation that allows a ZMA file (impedance/phase plot) to be used for a load, and then uses digital processing to emulate the circuit. Basically, it's a programmable digital crossover that you configure with a Spice model. Works great with my existing Spice models.Then of course, the original functions are still there to perform T/S measurements of a driver. Very easy to use. One nice extra feature is the ability to measure the speaker in the cabinet. After T/S specs have been measured, you can put the speaker in the box you've designed for it and measure the impedance in the cabinet. The WTPro/ST will then import this information into it's built-in box modeling program to plot response. This makes the box modeling program even more accurate than just entering box size and port dimensions because the WTPro/ST uses actual impedance measurements of the speaker in the box.
The WTPro/ST also performs acoustical measurements using several signals and methods. This allows you to get the raw response of each of the drivers, which is important when designing the crossover. After you've used all the design tools to optimize the cabinet and crossover you can measure the final results using one (or more) of several different signal types, including swept sine, MLS, noise, impulse or chirp. Gating controls are provided for pseudo-anechoic measurements (indoors). You can also do THD, IM and SINAD distortion measurements with the WTPro/ST. It's a pretty impressive system.
Check out these related threads on the Measurement forum: