We found this company on this forum. VERY professional. Driver was right on time with a very nice and comfortable car. He knew where to go and the fastest way to get there. Gave us perfect directions on what to do when he dropped us off at the great wall in Mitanyui (sp?) and after that he dropped us off at the summer palace. Again, great directions and advice.
The service is reasonably priced. No need for an actual guide, I think. For this trip, a driver alone is perfect and you can save cost. It was only approximately $120 US dollars.
I highly recommend Miles Meng if you want to go to the great wall. Make sure you add the summer palace to the day. If only there was no smog!!!!!