Panjiayuan is much more than just an antiques market (though that is what the English sign near the entrance proclaims). The vast majority of the stalls are not devoted to antiques at all but rather to a tremendous variety of goods, including Tibetan Buddhist and traditional Chinese paintings; jade (and fake "jade") carvings in all shapes and sizes; a wide assortment of all types of jewelry in both wholesale and retail amounts and prices; porcelain; lacquer furniture; silk pillowcases, table runners, and purses; Tibetan saddles, knives, shields, and clothing; Communist memorabilia, and many more unusual items like traditional Chinese paintbrushes, ranging in length from 6 inches to 3 feet, and Indonesian-style shadow puppets. My favorite thing to shop for is the embroidery and silverwork of the Miao ethnic minority, available in a profusion of stalls whose varity and quality surpass anything I found in Guizhou province, the Miao homeland. You should be prepared to bargain hard if you decide to buy, though I have found that the Miao women are less willing to bargain than many of the other vendors, often only accepting a 25-50% reduction on their initial offer, as opposed to 75% or more for many other stalls. Panjiayuan really does have something for everyone, whatever your interest and price range; in my opinion, it is hands-down the best place to shop for souvenirs in Beijing, and perhaps in all of China.
Practicalities: The main market area is only open on weekends, though there are a few rows of antique vendors there during the week. Arrive early to avoid the crowds and (supposedly) get the best deals. Panjiayuan is located in the far southeast corner of Beijing. The best way to get there by public transport is to take the subway to Guomao and then a bus (300, 967, and others) south along the third ring road for a kilometer or two; ask to get off at Panjiayuan.