Quoted from EvanDickson:I would hate these and peel them off any machine that came with them. I would find them cluttering. And artwork stuck on top of the glass would seem... out of place... compared to the art I expect on the machine playfield and plastics.
Also, for me anyways, anything that obscures ball travel in any way gets stripped out of machines that I buy, even if it's a really cool and well done mod and it's just a return ramp or something. With that in mind, something like these made intentionally for that purpose could be useful to handicap play on easy machines for tournaments, akin to the blinders on DataEast Tommy. But that would be a very small niche to fill.
I've also met people who don't think like me and would stick like 10 of these on every machine, so you might be able to make a go of it.
Thanks for the feedback. To be clear, they are not decals and can be removed as they are only held in place by static cling. So if you had a machine with one, you could take it off as easy as picking up a quarter off the glass. Or the former owner could just keep them and use them in other ways.
Also, my idea is to use them in places that do NOT block play, for instance, maybe a shooter lane or over a dead area. But as the owner, you would place them as you prefer.