In the wild of bike touring, there are too many freaking options for bags to make a choice easily.
Most people settle with Ortlieb because they are so visible to the consumer. But I would dare to propose, that Ortliebs are wholly inferior to another brand of bicycle bags.
Ortliebs have a lot of plastic hardware, that will break at a much lower stress than aluminum. And you can bend aluminum back into shape, Plastic will just deform... plastically.
Ortliebs are essentially dry-bags for bikes. They create an airtight container in which bacteria will grow mold, and now you've got a soup container instead of a bag.
The attachment system for Ortliebs is clever, but does not isolate vibration. Because there is no tension in the attachment system, they are free to bounce and giggle and vibrate on the rails. Now, the clamps themselves are very intelligently designed, Just lift up on this central strap, and the claws are disengaged from the rack rails. But the bottom mechanism is supremely asinine. There is a hook of plastic that keeps the bag from swinging way out from the rack. No tension cord. No spring loaded finger. Just a dummy hinge that keeps the bag from swinging too far. But it offers no vibration isolation.
Alternatively there is a Canadian company that is immensely popular with its customers. For it's value of an unequivocal, transferable, lifetime of the bag, unconditional repair/replacement warranty. But frankly, the bags are so damn tough, the warranty is really saying, "If the bag breaks after 20,000 km, well get you a new one."
They make all of their bags with non-waterproof Cordura; Now they do have a waterproof set of bags, but it's only a set of bags for those of us who bike in severely rainy conditions.
The Cordura they use is the same Denier (durability rating, the higher the tougher) as Ortlieb, but it's not waterproofed, so you won't start growing junk in your trunk. The Cordura itself is very water-resistant, but they also offer water-proof covers for every bag. Because a proof cover, is more versatile than a proof bag.
The attachment mechanism is just as clever as Ortliebs, but is more durable and isolates vibration at each point of attachment. The hardware is all high-grade aluminum with poly-something coatings at the clamps and a spring-loaded cam, that grips the rails of the rack. The bottom attachment is a simple hook suspended with shock cord to tension the bag to the rack. Again, it isolates movement of the bag. Something you'll be very grateful for on gravel roads.
I could spend hours talking about this company, but take a look for yourself, at Arkel Over-Designs.
The thing that makes them a better solution is their versatility, but most importantly their repair ability. Every bit, bolt, and structural component, in nearly every bag is removable and replaceable. This makes them very easy to repair when your in the middle of some town, with no bike shop in sight, halfway through your tour with no signal on you cellphone and some busted hooks on your bags. Because shit happens, and you need the ability to fix your equipment while bike touring.
I do not dislike Ortlieb, but I like Arkel's designs more. If the two wanted to collaborate, I would be Immensely excited to see what they could create.