I bought a JET chip when I first had my car and didn't know any better. What the chip does is increase fuel and spark, and eliminate the shifting timing retard... I believe thats basically it.
I had to run 89 octain, but getting rid of the shifting issue was reason enough on its own.
If I could do it over again, I'd either buy a TwEECer right off or get one of those slick 4 position SCT chips.
Adam