I haven't tried the other Peking Duck restaurants in Beijing yet, but I have been to the Quan Ju De near Tsinghua University three times already. Each time, the food was amazing. If you have four or more people and order a number of other dishes, including the duck, you'll only spend around 70-80 RMB per person and feel satisfied. The dishes besides the duck were also excellent, though I fail to remember the dish names (one being the fatty red pork). The atmosphere is nice, clean, and the servers have always been friendly. I haven't had any complaints yet, in the three times I have been there within the past three months. It's a great place to take guests. You do get the souvenir card for each duck you order and you can watch as they cut it up. I have to try the other restaurants in order to genuinely compare which is the 'best' duck, but even a local Beijinger has recommended this to me as the best.

** Edit: I just tried the Quan Ju De at Wangfujing. I believe the duck at Wangfujing actually might have been a little better than the branch at Tsinghua. However, the duck broth which is served at the end was more watery than the rich broth I normally enjoy at the Tsinghua branch. I also don't recommend the duck fried rice as it was merely passable. The branch at Tsinghua definitely has a better selection of dishes other than the main course of duck, and although it doesn't matter a great deal, this branch offers a nicer commemorative card than the busier counterpart in Wangfujing does.