Great addition for gravel races or centuries in changing conditions. Holds spares and provides a place to stash layers and food. Does make bottles a little hard to get to, but you should be using a bartender or feedbag on gravel, anyway. Size L fits 56cm Surly Midnight Special, 58cm Salsa Colossal, and Size L Salsa Fargo.
Excellent quality materials and construction. Very secure fit on the frame, it doesn't shift around while riding. Lots of room for spare tube, mini-tools, co2 andor pump, snacks, jacket, etc. This replaces a seat bag and more!
I wanted a smaller frame bag for my fat bike(Trek Farley, size XL) so I didn't need my hydration pack. Good size for spare tube, pump, multitool and spare windproof jacket, but it gets tight. Would be nice if it was a tad larger so I could fit an extra pair of gloves. I'm very happy with it and didn't want a full frame pack.
I use panniers for holding my clothes, computer and lunch when riding to work, but I was looking for something more easily accessible to hold a mini-tool, tube and pump as well as my phone, wallet, keys, etc. This holds much more than a saddle bag and is easier to access than panniers. I have found it to be quite waterproof. Only minor complaint would be that sometimes the ends of little straps that attach it to the frame come untucked and can be annoying when you leg hits it while pedaling.