We've had a lot of spam coming through the Postini quarantine in the last few months--a lot of fake Intuit/Apple orders, lots of viagra spam, I'm not protected by Norton, etc.
Unfortunately Postini already nabs some legitimate emails, too. So I hate to increase the level of filtering.
As a note, we subscribe to Postini's archiving, which means we have to keep using their quarantine service, as Google hasn't integrated archiving customers yet.