This is my second CoolCave Pannier because I destroyed the first one (More on that later slow).
This is a creat solution for carrying things you might not want to put in a soft bag such as tools, muddy boots, etc. I had a large u-lock tear through the bottom of a soft pannier a year ago and that is what got me looking for something like this.
I commute to work every day outside of the darkest days of winter and I have found that I can just dump my normal bike bag (Ortlieb Back Roller Plus) into this bag and pull it out when I get to work fast and easy. That way I only have one large protrusion sticking off the back of the bike instead of two. if I need to stop at the store on the way home I just pull the pannier out of the CoolCave, clip it on to the other side of the bike and dump my shopping in. This bag is big, easy to clean inside and out with a hose and it goes on and off the bike in a few seconds.
What’s not to like? Well, since you asked… The ascetic is pretty much like you took the trash bin fron under your desk and strapped it to your bike. I can see that would not be for everyone.
It catch’s a lot of wind at higher speeds. or when it is windy.
Finally, it is not quite as r=tough as you might think as I had to replace mine because I hit it on a steel bollard (Concrete filled metal pipe) swerving to avoid running over someone sleeping on the sidewalk. The bag hit the bollard and exploded, literally tearing itself in half and fracturing all over. I was not super happy about this in the moment (or the person sleeping on the sidewalk) but in hindsight it is not entirely a down-side as if the bag had held together it probably would have instantly stopped the bike nad sent me over the handlebars, or clothes-lined me on the bars, or worse yet, it could have pretty easily have deflected me into traffic. So, I like to think the bag sacrificed me to save me from my poor swerving skills.
I have seen reviews where they complained about the bag being noisy, but the only noice I have experienced is a little rattling on rough surfaces when the bag is empty. Most of the time I don’t hear it at all.
I also want to mention that while this bag is sold as an accessory for the Specialized GlobeHaul cargo bike, I am using it on an Lectric Xpress 750 with standard pannier racks and it fits and works with the included hardware.
I think this is not for everyone, but if you need an option to carry stuff that might not work in your soft pannier or backpack, this might be what you are looking for.