New York Times quotes a report from Lyris, an email marketing firm that tracked 57,000 mails sent from 57 businesses and nonprofit organizations. Only 3% of these messages were mistakenly labeled as spam by Gmail. In the first quarter of 2006, a similar study concluded that 44.1% of the business messages were sent to the Spam folder, although customers chose to receive them.
Gmail's spam filters are adaptable, so they get better over time. In 2004, Slashdot asked "How good is Gmail's spam filter?" and someone responded:
"So far, no spam whatsoever has found its way into my inbox. However, the amount of false positives filtered into the spam folder is overwhelming.
For a while I wondered why I only got reports by email about 30-40% of my finished online auctions (link omitted, no free advertising here). Last week I accidentaly clicked on the spam folder, and there it was, dozens of FALSE POSITIVES."
1 hour ago