I don't have the HA rack, but I do have the backobike bags with the same attachment system. It works very well, as you have discovered. However, the original web straps with buckles that came with my bags (the red, above) can be susceptible to fairly rapid wear. My theory is simple: square peg in round hole, does not work so good. I noticed fraying on my straps at the contact points with the two holes after a couple of months of riding every day, approximately 80-100 miles a week, fairly tame trails and road, but enough bumps and holes.
I purchased replacements from Rivendell. The green replacement straps appear to have a tighter weave, and are wider (and unnecessarily longer). I have not had them in long enough to notice a difference. Before ordering, I substituted a strand of climber's rope, which worked fine and is rated for a hefty load. Looks a bit more Rivish as well. Bottom line: I suspect this could be a potential failure point over time, but is easily monitored and addressed. I really like these bags, for reasons I expressed in another post.`