They are ok, but they are over-rated to be honest.
Lets put it this way -
When a manufatcurer builds a car, the engine thats made and put into each car may indeed be identical models eg 20XE but none are identical and you get good ones and bad ones.
In order for them to run, the manufacturer must make an ECU unit that will run ALL engines whether they are bad or good versions of it. They need to build only one unti so as to keep costs down. This means that the standard ECU is an "off the shelf" product thats not specifically tuned to the engine its mated with in the car and so fuelling etc are not ideal.
The Superchip is er......hmmmmm..... another "off the shelf" product thats designed to run with the same particular engine but do it all a bit better which gives you the increase.
If you use something like a UniChip, then this is setup for each INDIVIDUAL engine so as to match timing, load and fuelling ratios exactly to its engine ie at 400RPM intervals and at 3 different loads for each one. Thats why you will always get a better performance increase in a mapped chip than a universal one.
Its not that Superchips are bad (they do give an increase), its just they cant give as big an increase as an individual chip.
Rich