There's disagreement among scientists on this point, but that's not the same as concluding it's "ineffective."
https://www.theguardian.com/world/2020/may/04/scientists-disagree-over-face-masks-effect-on-covid-19
Once again, the virus is new and we're still learning about it, so caution is warranted. So this is an especially good time to reiterate the principle that absence of evidence is not evidence of absence. Further, it's recommended by the CDC and other world health agencies.
https://coronavirus.ohio.gov/wps/portal/gov/covid-19/checklists/english-checklists/cloth-face-coverings-covid-19-checklist
What level of scientific proof are you seeking before you take such a ridiculously easy and trivial step to protect yourself and others?
Have a mask? Wear it. Love it. Simple.