Im with Gear here....i was asked the same thing a while back, same product and told them i wouldnt do it. As you said, you "were familiar with my own products that are used on a regular basis". The fact of the matter is someone had hyped them up to using that particular product, and who knows what expectations they have/had. I would have assumed if they had used it before, they wouldnt be calling you?
Most of us have quite an array of detailing products, at least I do. Off the top of my head, I probably use at least 6-7 different labels, but am very loyal to my polishes and sealants, of which i use three of each different brands. I get excellent results, and satisfied customers, and an occasional ego boost. Why jeapordize that ?