The spam filter for Gmail works well, sometimes *too* well.
I have not checked my Spam folder for about a month, and yesterday I decided to open it. There were more than 20 legitimate emails that got stuck in there, marked as Spam. One email contains a large order made to my company, with proof of payment by the buyer (I had no idea at all about this order otherwise).
I spent a few minutes replying to these emails, apologizing profusely for the delayed response.
Bottomline is: Check your spam folder regularly (I plan to do it once a week from now on) and then clear the folder. Better still, set Google Calendar to remind you to do this once a week.