This is it in my car
It's really useful if you can tune your own car, when it changes I know to log the car and see if the tune needs to be brushed up before I rip on it. As I tweak the tune I save a copy and then can just load that tune up according to the display.
I think one of the things holding people back from signing up is getting a hold of a GM fuel sensor. I grabbed mine off ebay for $100 in the spring. I've seen them go for anywhere between $80 and $250 on Ebay for used/new ones. Finding a wrecked flex-fuel truck would be the best. Or if you are a baller you could just order a new one from gmpartsdirect.com for $360.
I also have a signal converter on mine that outputs a 0-5V analog signal, hoping to get that working with a hacked Evo9 ecu rom to alter boost/fuel/timing based on the Eth % like the new Saab turbos do. I can get the signal converter boxes too, they are $120ish