I found that running Spamassassin is way more reliable than Google's filter, although it missed some spam from time-to-time I don't think I ever had false positives. Meanwhile I can attest that the few times I tried to use Gmail for receiving something "important" (e.g. an invoice for an online service/shop I just signed up for) it went straight to spam. Maybe because there are so many spam invoices it just can't tell the real ones apart anymore.