We use both VisVSA and CETOL were I work (QMC).
We have expert users for both tolerance analysis packages.
I worked for VSA for 8 years and used the VSA software for over ten years.
I have used the CETOL software tool for the last 6 years to conduct tolerance analysis studies of large complex systems. If you have any questions about the application of either package please contact me at www.qmc.com.