Lonely Planet recommended Beijing Hikers as "an excellent way for visitors to get out and see the more difficult to reach and remote sights."
Thanks, Lonely Planet!
UPDATE 2014: This was a long time ago, but we're still an excellent way for you to get out and see sights that are more remote and difficult to reach. Even more excellent than before, actually! Our website address has changed since then, and for best results please call the office number (+86 10 6432-2786) instead of the mobile phone listed.