Well, I don't exactly know what the limits of the panniers are, but I put a 21-lb jug of cat litter in one, and it held but I think that's the max weight I would put in it. In the other pannier I had a bottle of laundry detergent and assorted other things. Certainly I had 35-40 lbs of stuff in the panniers. One thing for sure, it is hard to balance the bike while packing it, especially when something heavy goes in, because then it just wants to fall to that side. Sometimes I take the panniers into the store with me if I've ridden my bike to the store, but today I rode the bike to work and took the boss's car to the store. In this particular case, I kept my panniers attached to the bike because wrangling a pannier onto the rack when it is packed with 21-lbs is a bear. So far I don't see any wear or stress on the panniers, but I don't carry heavy loads for long distances. 3 miles is max.
Still nice to know that I can cycle just fine with all that weight back there.