I've gone over limit with 5 bags of river rock (about 270 lbs total) plus 30 lbs for the giant cargo box, plus myself (180 lbs)
I wouldn't do it again. Felt totally abusive and dangerous. I was glad when I got home and apologized to the bike profusely. Promptly checked the wheels out also. It took it in stride, but I was nervous.
I feel comfortable with <200 lbs cargo while riding slowly.
I feel very comfortable riding along with <150 lbs of cargo. I go easy on the brakes.
I ride with 100 lbs of cargo with blatant disregard for the bike.