People these days with any sense don't use them as all "tuning boxes" like this are complete rubbish 
wrong, can get ones nowerdays that are just as good as maps just a little cheeper.
yes you can get pigy back boxes that are crap and there usually around £50, but decent ones that are more closer to the £200 bracket are just as good.
the old cheep crap ones just fool the ecu into over fueling, you can get proper ones nowerdays that adjust fuel, timing and boost pressures properly.
cant comment on the one pictured above though.