The main features are:
Graphical.
The data can be plotted for each immitance types (Z, Y, C or M), in the complex plane, as well as their real, imaginary parts or their magnitude in function of the frequency (Log/Log scales).
Simulation.
The possibilities are almost infinite. Over the classical R, C, L electrical elements, one can use CPE (constant Phase Element) and Diffusion Impedances.
Fitting.
ZfitGUI makes use of the simplex algorithm. To fit only portion of the spectra is user-friendly. Note that there exists the programmatic version of ZfitGUI named "Zfit" which can be found in the Matworks site file exchange.