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I used my bagman on the Great North Trail in August with a Nelson Longflap with tent and all the trimmings. No issues on rough descents.
About 10 years ago I once had a screw work loose during a ride (not the centre one but one of the side ones) and the thing disintegrated onto my back wheel. But I check the screws are tight from time to time now and it has never happened again.
I've also used a rack for support plenty of times. I think the rack is actually slightly lighter and more versatile, but I prefer the bagman if only hauling Carradice as, especially if there is not much in it, the bag can sometimes slip down at one side on a rack.
I think modern bagman models (bagmen?) have fixed most of the weaknesses.
Mates that have the SQR for commuting duty seem to like it, with two minor reservations: with long use it can start to rub/wear the back of the bag, and to get the bag on/off it needs to pivot slightly so it drops, which means it may not work for small frames with not much seatpost showing.