Superchips are remaps of standard ECU; the guy in the garage connects a wire to your cars diagnostic port (near the gearstickk somewhere) and "flashes" the new settings into the ECUs memory. They can be reflashed with original settings just as easily.
Ive had mine done, and its OK; pulls harder lower down, feels slightly more breathless after about 6500rpm than it did with original ECU settings, so consequently theres little point revving the nuts off it. I dont reckon its given me anything like 12bhp, probably more like 2 or 3 to be honest, but torque-wise it feels 10-12 lb ft stronger between 3500 and 5000 - it now pulls at those revs like my VR6 used to.