just wondered if these are worth getting? also, anyone know how the things are fitted?

M

 

It is a one dimensional adjustment of your fuelling and when I built my own it exacerbated a lean area low down and killed spool up but assisted top end power. A bit unsophisticated. There are "super" versions which give more control but still very poor resolution. I got fed up with all this crude piggyback stuff personally, but they do have a role on some cars.

It interfaces to the MAF signal and adjusts the voltage.