I have an S-AFC 1 on both my cars. They are only useful when you have altered the air flow into the vehicle (induction kit or the like). On some vehicles, adding an induction kit changes the air flow beyond the constraints of the original ECU. Often the ECU then works the injectors overload....as a default setting hence the engine will run rich. The results of adding an AFC are dependant on other mods and the vehicle you are installing on. Power gains can be 0 ....sometimes they can be very good...
One thing I would advise is to have it setup on a rolling road.
Apexi can supply default settings for your AFC in certain applications (such as AFC with an APEXI induction kit on car X use these settings.......xxxxx). I followed these settings for an Altezza RS200 and gained 3BHP, took it to a rolling road and they gained 12BHP.
Just my 2pence worth....hope it helps.