I am currently really thinking of giving postfix a try, but it somehow "feels" like stepping backwards.
I am really interested in some experiences of postfix admins and how they handle virtual domains and users.
Postfix is great, efficient and easy to use. Also there are lots of plugins developed for it. For 10k mailboxes it will run smoothly. Back in 1998, we had a postfix server running more than 50k accounts on an old Sun Ultra server, and it was not causing much trouble. Well, the users were of course mostly dialup ones, and spammers weren't that active that time...
Comparing to qmail, I'd advice to go for postfix.