Fast SPICE?

There are some Fast SPICE products out on the market which purport to
allow accelerated SPICE simulation times in exchange for somewhat
reduced accuracy. They are used for extra-large chip designs, where a
real SPICE run would take far too long to be useful.

I was wondering if folks in this group had any experience with Fast
SPICE and could offer comments about its usefulness and accuracy?

Thanks for your insights!

Stuart