I have a whole bin full of seat bags that aren't quite right. The Mattone on the other hand has a permanent place under the seat of my road bike. It is the right size, has durable construction (so far), has a quality zipper, and looks good doing its job being awesome.
If it had a sewn-on velcro strip on the back to easily attach an extra tail light and/or a reflective strip this would be the perfect bag in its class. I tuck a reflective strip into the strap but it would be nice if it were an integrated feature.
Still, I have nothing bad to say about this product. It is a quality piece of kit that I strongly recommend.