This pack is used for one or two day bike packing adventures. I mount it to a Specialized Diverge 58CM with Apidura's saddle pack and top tube pack. Very durable and well made. I have not used them in the rain so I can't speak to the water resistance. The grey color is a nice accent to my bike.
Well-designed bag for small loads such as tools, a tube, and other incidentals. It fits rather flat, so will work fine under a jersey, but the flat design does limit it's carrying capacity. A stretch pouch outside the main compartment will fit a road tube, no problem, though a MTB tube is probably too large. Hipsters, if you want to carry a can of PBR with you, look elswhere. Ditto CCW types, your compact 9mm is too big for this bag.
Despite the smaller size, I find this bag very useful for many rides, to carry my phone, wallet, and tool kit, leaving jersey pockets for food, and a wind shell. Also good for summer use, when it's great to have nothing in your jersey, so completely unzipping for a Flapper is easy.
This is the second one I've purchased. As I stated above, the top tube pack is light, well made, and stylish. It functions very well at the other end of the top tube fastened to the seat post. The only consideration to be aware of is if there is not much space between the headset and the stem you may have a hard time attaching the front strap as it is wide and positioned in the center.
Perfectly watertight for well more than my needs, and the Medium has plenty of space for tube, multi tool, tire levers, patch kit, a blood sugar test kit and glucose tablets for this diabetic. If you arent carrying the diabetes stuff, a packable jacket can be crammed in there.