Google had no need for Postini. Google's own spam filtering in Gmail is pretty good.
In my experience, spam filtering in Gmail is pretty BAD because of large numbers of false positives. Its algorithms appear to have some kind of quota for spam, and if your account gets no obvious spam, they will pick some for you out of incoming newsletters, new correspondents, and legitimate commercial email. GMail is the only email server I have to check daily for false positives.