I used to go on hikes with Beijing Hikers on a frequent basis because they are well organized, they take care of all logistics, and the guides instill a certain level of confidence when navigating sketchy terrain. However, when I met up with one of our other forum regulars last year, he advised me that there are numerous other companies/groups that do the same drill at lower price points. Regardless of what outfit you use, I agree that it's a great way to explore the outskirts of Beijing, including lesser known GW sections.