I have two identical frames with baskets. The first is a Wald 137 but I modified the struts to attach to a low rider rack mount. Yesterday I carried a 7 kg. turkey and two bottles of wine and a can of cranberry sauce. Not only was it difficult to steer, I thought the basket brackets were going to bend. It was only about a kilometer home so I went slow but I could feel the basket moving around, won't be doing that again.
The second bike, the one I should have taken has the same basket with a Gamoh Porteur rack to support it. I am sure I could carry another person on that without any problem. The one drawback as people have already mentioned is the weight of the rack. The basket itself is very light.
I dislike back racks and would never use one again. If I have a really large load I just use my Burley Travoy.