I highly recommend John Ping. He is prompt, friendly, funny, knowledgable and speaks English well.
We hired Mr. Ping for a trip to Mutianyu (Great Wall). He told us various things about Beijing along the way, and recommended itineraries for the sights we wanted to see. It was our first day in Beijing, so this was greatly appreciated! At the wall he recommended adventuring on part of the undeveloped wall which was a great little escape from other tourists. We had heard there were many fish restaurants around this section of the wall and asked if he could take us to one for lunch. He took us to a fabulous place where you catch your own fish and they grill it for you (delicious!). He also ordered a few other dishes for us. We would have been lost without him as everything was in Mandarin with not even a Pinyin word to be found!
I've seen a few reviews commenting on his driving. I found his driving to be very normal for Beijing. It might be a little crazier than you're used to, but it's no different than any taxi ride you'll have during your stay. Just keep an open mind and remember driving in Beijing might be more agressive than you're used to.