Mine survived a fairly heavy fall with only slightly bent frames and scuffs. I'm still using them without any repairs, although one of the plastic mounting bits is also slightly damaged.
However, my friend just caught one of his on the bumper of a bus in very slow traffic and it made a right royal mess of the pannier and mounts. Bolts rather than the standard self tappers would probably have made things worse as the pannier wouldn't have been able to pop off.
At the end of the day, they're fairly thin aluminium which compromises the strength, but I feel the light weight more than makes up for this.
On a big GS, if you damage them too much to repair you're probably going to have lost interest in the bike too!