When I first got the cubes, the material seemed a bit flimsy and did hold its shape. However, upon using them every day, I find they are perfect for my needs as the flexibility of the material allows me to be flexible with what I pack. The larger cube holds all of my clothing needs, the medium is for my tools, medical kit, etc, and the smallest is for snacks. Perfect! You could probably come up with your own version of pannier bags. But because these are specifically made for my Ortlieb panniers, they work better than a makeshift version.
I bought this for a bike touring trip but sadly, it arrived the day after I left! That said, it is an attractive bag and seems well built. I like the fact that it has a solid insert to hold its shape (unlike my partner's bag) and the inner zip pocket is convenient. The lid is easily opened with one flick yet is secure when closed, so is a nice balance. It is also light, and doesn't 'dominate' your handlebars.
I have two minor critiques. First, the insert is helpful for holding shape when carrying something heftier in it: for me that is a camera, but the bag seems a bit small for carrying a modern DSLR and mid-range zoom (with a small prime lens instead, or an older film camera and mid-range zoom it is more than sufficient). It fits, but eats up most of the space: get the 7-liter if you want to carry such a set-up. Related, the insert could be a bit more robust but that would add weight. And the bag should include the clear top for inserting your printed itinerary.
Overall a very nice bag. I used Ortlieb Backroller Plus panniers on the journey and they were flawless. Like the build and design of the Ortliebs: I see why these are the most famous cycling bags. Should I size-up (I hate upgrading -- buy right the first time -- but I have several bikes so it is helpful) I will likely get the 7 liter version.