As they say, Works For Me. Admittedly, I don't know how many important emails I've lost over the years, but I don't remember ever being contacted elsewhere about an email that turned out to be in spam. I have a bad memory so I might be forgetting some instance, but if so it must not have been terribly important, or I'd remember it… On the flipside, there have almost never been any false negatives. I remember being quite annoyed a while back when there was a brief period where a trickle of spam got through. Other than that, I've been spam-free.

(I also remember being IMed by spambots using XMPP federation a few times.)

This does come at the cost of letting Google see all my email. I'm not happy about that, but I'm skeptical that a fully encrypted solution could provide filtering as effective. Certainly a self-hosted solution cannot.