Gearjammer before my Austin trip as I wanted to avoid carrying a backpack while scooting around town. I've been testing my collarbone's weight bearing abilities, but having ~15 lbs strapped to my back seemed like a bad idea. This Gearjammer is what Oveja Negra refer to as their large bag (8-12L), though it is still smaller than other options out there on the market including Porcelain Rocket (6-13L) and Revelate Designs (6-14L). I don't think 1-2 liters makes a huge difference in usability, but sloppy packers should take note.
The Gearjammer's base at the seatpost is an HDPE shell with extra padding all around. This helps keep its shape and also give the bottom of your load a place to fill and give purchase on the seat post. Without this plastic reinforcement, the bag might sag or bulge and shift around the seat post.
The Gearjammer attaches to the bike both at the seatpost and over the saddle rails. The seatpost/bag connection is buffered with extremely durable and grippy Superfabric. This keeps the main bag material from taking the brunt of friction, though this bag doesn't move up and down as it's held by the Superfabric and two velcro straps. Possibly the most ingenious construction decisions come from the locking Nexus Cyberian SL buckles which keep your connection with the seat rails in place. It's really impossible for these buckles to come loose and for the strap to slip. I will warn you though that if the bag is minimally packed, you can't get the bag snug enough under the saddle and it can sway.
I certainly didn't pack this bag to capacity. At most, I had an extra shirt or hoodie, a few beers, my U-lock, a Ramblin' Roll, and some energy bars. I've no doubt that you could stuff a packable down vest and tightly packed summer sleeping bag in there in addition to these things. This would also make the saddle rail straps fit more snugly and keep the bag from swaying. It benefits from overpacking.
This particular bag is similar to others on the market, in that it has a rolltop opening with a buckle closure on either side of the opening. This enables the load to expand as you take off layers during the day or acquire things on your ride. Conversely, the bag can pack down smaller if you end up taking things out. This is noted by the smaller capacity in the size designation. Fastening the side buckles and synching the load is easy thanks to long straps and toggle webbing that you simply tug to bring them taut and pull on the buckle/webbing junction to loosen. Another buckle holds the rolltop in place around the outside at the back.
One feature I've yet to utilize is the MOLLE system on the underside. I would imagine that bungee cords and spare pockets might bring this bag to the top of the heap with regards to versatility in a pinch. Also note the plastic loop at the top of the MOLLE strap. I'm not entirely sure what this is used for, though I would assume it can assist in making you conquer a load that is simply too large for the bag.
We've come a long way from the days of overstuffed messenger bags. With options for loading the bike at the saddle, frame, panniers, and handlebars, there is often no need to strain your back with a load. I'd definitely give this Oveja Negra credit for its ability to handle a load without too much input to handling. Generally, the higher the load, the more your handling will be effected. As long as you make sure you put the heaviest equipment towards the bottom of the bag, you should be golden. Retail on this bag is only $120 and can be purchased direct via Oveja Negra.