Yes, I attended a marriage interview with my clients in Sacramento a couple of months ago. The officer did not decide the case on the spot, but we received an approval by mail a few weeks later. Of course, if the officer plans to deny the petition, it often takes a few weeks or months to write the Denial or the Notice of Intent to Deny.
This is becoming increasingly common--most officers now are ending the interview by saying "We'll mail your decision," even if the decision is favorable. You will usually hear within 2-3 weeks if there is an approval; longer than that generally means that the case has been referred to a supervisor or that additional information is being sought. If you haven't received a decision after 90 days, do schedule an InfoPass (at www.uscis.gov) appointment to see what the case status is--the InfoPass officer can contact the officer who conducted your interview to ensure that the case hasn't gotten lost in the shuffle.