Jet chips and programmers?

I'm considering buying a couple of Jet products for my 94 GMC dually and 2001 Astro Van.
For the dually, Jet offers an under-dash mounted unit called a "Six-Pack" that has six operating features, four of which are performance choices.
The Astro is OBDII with a Vortec 4.3 for which Jet offers a programmer that can also calibrate the speedo and program transmission function...

If anyone has experiences (good or bad) with these products, I'd be real interested in knowing what to expect before plunking down the bucks... Thanks!