Oh boy, you'll get answers all over the board here.
I've got one on my truck that I installed with the CAI. It relocates the Incoming Air Temperature (IAT) sensor from the MAF to the remote location. Vector's kit is supposed to be a quick response unit, meaning it's faster to respond to cooler incoming air and take out timing retard.
I'm very pleased with the way my truck runs whether it's dead of summer or otherwise, so I think they work. When I hit the loud pedal, it responds.
Another big reason for getting a tune is to clean up the lazy 1-2 shift at light throttle and make the trans shift like it ought to for a performance vehicle.
That's my two cents.
06 Black LT 2WD (original owner) - Vector tune, CAI & HSRK, Alvin's trans cooler (brake duct mount), braided stainless brake lines