So... I took the plunge last week and ordered one of these. Occasion was a finished mini which I completely painted OSL (a ranger holding aloft a lamp) and I figured the effect would be best visible against a really black background.
So the day it arrived, I took it out of the box and immediately put it to use. However, the results were nothing short of dire...

It would not register as true black on my phone camera, but worse; it appeared to contain loads of little white flecks! Now, I'm not the best house keeper in the world, but I do keep my hobbyroom clean and that piece of cloth was literally a couple of minutes out of the box when I first used it!
So the way I see it, there are three possibilities;
1) I suck at photography and no fancy background can change that (a very plausible option)
2) I was sent a used or returned item that already contained all that dust etc when I opened the box
3) Both of the above
Mind; when I look at the sheet with the naked eye, it looks absolutely pitch black and those white specks don't even register. But when I use my camera(phone), it looks like a frickin' night sky.
I am using a daylight lamp and two LED units that are significantly more red. Would this have anything to do with it perhaps? Do I need to diffuse the lights maybe?