Pretty effective.
In Kansas, the governor enacted a mask mandate but allowed counties to opt out. Counties that enforced the mandate saw a decline in infection rates; counties that opted out saw cases continue to rise. https://www.cdc.gov/mmwr/volumes/69/wr/pdfs/mm6947e2-H.pdf
Now of course masking is just one NPI that works. Combine it with distancing, tracing, testing and better hygiene and you would see a very rapid improvement.
Sure, they'd be better. But they are also a lot harder to use, a lot more expensive for an average person to wear and impractical for many at-risk people (dental hygenists, cops, teachers etc.) But cloth masks are sufficient to reduce infection rates, and that's the goal.
Most people aren't assholes, which is why this doesn't happen very often. (Of course, a few are, as swatting cases demonstrate.)
Didn't you JUST GET DONE telling us all that you should encourage people to do the right thing instead of counting on the nanny state to enforce things? Like an hour ago? Make up your mind, man.