The good news is that this medication has been used for decades to treat malaria and other ailments, so we have a lot of experience with it and it is generally safe. Also, it is produced in mass quantities in generic form, so there should be plenty of it.
As for efficacy, there was a study done in France that showed great results, especially when given along with an antibiotic. The problem is that the study has a very small sample size and was not a blind study. I’ve also read that it has been used with some success in South Korea.
Since the medication has already been approved by the FDA for other illnesses, doctors in the US are free to prescribe it “off label” to treat coronavirus. I’m sure many will do so in the next few weeks, at which point we’ll have a lot more data about its efficacy and safety.
So it’s a promising development but too early to say just how helpful it will be.