Was at Duck de Chine last month and it was very good. Of course, the Firm we were with booked a private room and all that jazz, and personally I thought the setting/atmosphere was the best part.
I still think that Da Dong made the best impression on me duck-wise (the other food is very good to - unlike Liqun) closely followed by Liqun (great for a hutong stroll before/after the meal - killing 2 birds with one stone).
What I need to do is to get a group of friends together one night and hit all three places and just have some duck at each place to make a comparison.
I tell people to avoid Quanjude - the duck is quite good, but its too crowded and too commoditized in my opinion.