There are a couple threads in this...
Its a piggy back contoller, meaning it intercepts the MAF input signal to the PCM and allows you to modify it in 200rpm steps to compensate for an add or drop in fuel requirements based on modifications and desired A/F ratio.
Good:
-Easy to install
-Easy to operate
-Allows you to monitor various sensor values and voltages
-Great tuning tool on a dyno
-Works best with a Calum or JWT tuned ECU as a "fine tuning tool"
Bad:
-$300 new
-Not too helpfull with just basic mods
-Difficult to tune with a wideband O2 sensor
-Needs a dyno to be truly effective
Try searching if you want to find more but that should get you started
Jeremy