Really easy and quick.. you didn't get stuck in traffic jam.. as long as you know where you are going its the best alternative!!
Beijing city is served by 3 lines of underground.
Tourist places can be reached using this transportation: tiananmen square, forbidden city, Lamas temple, heavens temple [not too far from station]
Easy to use and cheap,but usefull only for Silk Market and Lama Temple or Tianamen Sq. in my opinion.