This is probably one of my favorite duck restaurants in Beijing. There are more famous and more expensive, but they don't compare. While maybe not Beijing's best duck, Xinbaiwan is definitely a contender. Also because they are away from the touristy city center, they are cheaper and almost exclusively Chinese, although some backpackers do stumble in on recommendations from the staff at 9 Dragons House hostel (which is how I found out about it two years ago).
The food here is good, traditional Beijing cooking. It's affordable, 90 RMB for a whole duck, compared to 190 RMB that some places charge. I also recommend the eggplant and green chilies, although it is only for those who like their food really spicy. The restaurant has an English menu so you can point at what you want, although bring a phrasebook just in case, as the staff speaks no English. Two people, maybe even three, can have good meal for 200 RMB. The restaurant is open 11am-2pm and 4pm-10pm, seven days a week. To find it, take the line 5 to Zhangzizhong lu, leave through exit C, and take an immediate right. Xinbai wan is right there.