I think they're a neat idea, but I don't believe I'd have much use for one personally (I don't detail for money). It could serve as a wow factor for a customer though. Just be sure you don't have one customer who goes toting their car has 95 gloss units to find out their friend has 97 and their detail was less.

The customer has to realize that when you compare one number to another, it would have to be from the same car at the same spot. Comparing readings from one car to another is nearly impossible. At least that's how Loach's post would make it seem. On the same note, would a customer feel cheated if you detailed their car twice and had less gloss units the second time? The results may be the same, but are your gloss meter results repeatable?

Loach, for test panels, could you get sheets of 1/4" steel plate and have then primed and painted for a consistent test pad? I've heard of people painting the backs of backing trays/sheets for tests, but worry they may flex too much for your needs.