[Unknown LJ tag]
My biggest beef with qmail is djb's stupid license and the mess it's created. He refuses to maintain the software with features that users want, so there's a venerable sea of patches you need to apply to make qmail useful. What an aweful mess. It's also a lot of fun having 18,000 bounce emails in your queue because qmail blindly accepts email for accounts that don't exist.
Anyway, the upgrade to postfix was pretty painless. I firewalled port 25 except for localhost and a remote machine (that's *not* my secondary MX!) while I was migrating, so I could properly test everything without the risk of losing legitimate email. Postfix now supports all the virtual domain features qmail has (and more), and that was the only area where qmail was stronger.
Now I just need to setup my secondary so that it can reject emails to bogus addresses.