No problems so far with my mounts holding up. As you can see in the above pic my pannier gets stuffed - what you can't see is the 15" laptop, papers, files, textbooks, spare clothes, and a myriad of other things filling it up. The "feet" sit flat against the dropouts on my bike, and the longer bolts sit well within recessed holes on each "foot". It's pretty solid, and while I don't go looking for curbs to drop off of, I'm not afraid to do so.
I respect Surly's point of view, but IMHO the Topeak system works well. Surly's disclaimer warns against "jury-rigging", but the Topeak rack is hardly that; it is a well thought-out piece of equipment. That said, I wouldn't expect Surly to warranty a broken mount that was caused by a rack.