Marcus' experience was that the laptop pannier he chose (the Axiom "Transition") didn't have an obvious (or documented) lock-down mechanism to guarantee that the pannier wouldn't fly off the rack. (His dispute with the Axiom company is that they did in fact have a locking device, but failed to inform the users about it! I can't blame Marcus for being heated.)
My big panniers sit far back on my rack, to make room for my big feet. They are secured with hooks, with heavy elastic tie-downs, and (as a backup) with Velcro safety straps that encircle the top rails of my rear rack. (See the image.) If the panniers that you use on your bike have a comparable safety device, use it, even if it takes an extra moment to do. (Marcus will tell you it's worth it.)
Although these are by no means expensive bags, (they're the Nashbar house brand), they get the job done. They are plenty big (and that's a major virtue), and they are soft and kind of floppy, so that abrupt bumps are "suspended" by the flexibility of the bag itself. I've hit many bumps over the 3,000 miles of last year, and no major accidents yet. It's also a major advantage that, if I park my bike, I can easily lift out the backpack with my laptop, and carry it with me comfortably.
I do consciously think about my bags. I look back at them at least once on every commute. If I see them swinging about, I stop and check attachments. So far, so good.
PS: End of the first week of March, and I'm now at 500+ miles. Woo-hoo!