Interesting debate. We are members of the Good Garage Scheme and have had no adverse comments about additives we have used. But I do agree that the customer must be made aware that the additives will be used, to that end we display a sign informing them of this and they are at liberty to refuse them. To date no one has!
The Good Garage scheme may not be ideal but it is an attempt to bring independent garages under some sort of regulation, which is needed to weed out the cowboys and before the government get all heavy handed with us. We give every customer a feedback form on which they can have their say and rate us accordingly, the results are then posted on the GGS website.
As for the additives, we have tried them in 3 of our own vehicles (because I was sceptical too), the first one with the forte rep present and the others when he wasn't. They all showed improvements in compressions and emissions and the oil stays cleaner longer so we can say to customers, hand on heart, that we recommend them.