I ran qmail for more than a decade I think (after sendmail), and don't regret it. But eventually ended up with postfix because I got tired of chasing patches to get qmail to play ball with the evolving, modern email world. At some point, the risk/reward equation inverted.
the problem of qmail is it's author DJB. He considered it 'done' which obviously isn't true. There are quite a few patched versions around but no fork got enough traction to become a living and maintained project.
For a long time you couldn't actually fork it because qmail didn't have a license and djb refused to add one due his unconventional views on licenses and his general stubbornness. So the only thing you could do was distribute a "patch set". And all of this also meant it wasn't packages in many repos.
By the time he finally added a copyright notice it was kind of a "too little, too late" kind of affair.
This is the story with most "djb-ware": daemontools, djbdns, qmail. I think it's a real shame because all of these had great potential to be picked up by others after djb himself lost interest. I suppose daemontools is the most "successful", but only in the form of the runit re-implementation.