I had these in 39mm and there was no problem reading the text.
Also, as for uses, IIRC, the story was a dealer at a card room had a few of them in his rack... When people would allow the action down because they weren't paying attention, he'd toss one at them and the table would laugh.... The dealer got his point across to keep the action moving.
They don't have a 'use' the way a other buttons do... Just kinda a gag.