I do value my own time, and I'll say it again – post setup, I really spend no appreciable time keeping my mail server running. My VPS provider handles all hardware issues (with zero downtime in the last two years), backups consist of a nightly cron job and require no manual intervention, and the last time I touched my Postfix configuration was over a year ago.

"Rolling my own filtering" consists of placing a single mailbox_command line in the Postfix configuration file, it hardly qualifies as a time sink. Writing mail filters for procmail takes no more time than doing so in Gmail, the difference is that the former lets you write much more expressive and fine-grained rules.

And yes, SpamAssassin does have a built-in learning Bayesian classifier. Which I previously used to learn from anything that I manually classified as spam, but these days I don't even get any spam to invoke it on. See sa-learn(1) for details.