I love using this saddle bag, the 0.5 liter size is just big enough and the mount is much better than Velcro straps - I no longer have to worry about my seat post getting scuffed up. I have a Cambium carbon saddle and it is a little tricky getting the mount to stay in place but not that big a deal, I was just surprised at how much I had to tighten the screws. The convenience of this saddle bag is a bonus, being able to slide it out of the mount rather than undoing Velcro straps is quite nice.
I wanted an easy on/off bag big enough to hold all my stuff. This bag clips into a separate mounting bracket attached to the saddle. Easy one handed removal and replacement. Sturdy, will not accidentally fall off. I carry a gravel sized tube, CO2 inflator and spare cartridge, tire levers, disk brake removal tool, hex wrench multi tool, and a small cable lock with room to spare. The bag does not have a zipper closure. It's waterproof. The opening folds over and is secured by two elastic straps on either side. I have broken a few closure zippers in the past. This is a much better way to secure the opening. It's waterproof. I have never owned a waterproof saddle bag!