Just got back from our Beijing vacation and although I had read alot of articles about not needing a tour guide in Beijing, I used one anyway. This is just to give my personal experience.
We have never used a guide in the past but felt that considering the possible language barrier, we researched and found a guide.
There are posts in the forum about the best guides and I chose one based on e-mail responses. Her name is Vivie Pan (email@example.com). She has a great command of the english language and a wealth of fascinating historical information that was not in tour books. She did not "push" shopping trips and warned us of scams. She was concerned with our well being. She kept our itinerary flexible and even gave a cell phone for our use while in Beijing. She arranged for my wife and I to get Taichi lessons at the Temple of Heaven and took us on a trip to buy tea from a wholesale place in Beijing (there were no tourist there only locals).
If your going to Beijing and want a guide consider this very capable guide.