Seems well made, as I'd expect from Castelli. Had to laugh at the XL name, though. It is XL compared to other saddle bags in the same way that Castelli XL jerseys are XL compared to most other brands--in other words, not really XL at all.
Packed fairly tightly, i was able to include a road tube, two CO2 canisters with an inflator, two tire irons, a multi tool, and a cloth mask. The bag fits very snuggly under the seat and looks pretty good. For me its size strikes the right balance between what I want to carry and how big of a bag I want under my saddle. If I flat, though, I will have to unpack and then repack the entire thing in order to get the tube out.