I needed something just big enough for a few tools and wet weather gear on rides that didn't need a top box and panniers. This is a superb choice. It is expandable and looks good on the bike. Inside is a dry bag, the sort that folds over and completely waterproofs its contents. Fantastic! The problem for me is, the dry bag is sewn in to the outer and takes up a lot of valuable space. As I intend to stow tools and waterproofs, I would forgo the dry bag for a bit more space. It does incorporate elastic toggles in each corner to hold the dry bag wide open, which is OK ... but, when I am sure of my needs I may well cut the inner dry bag out (hopefully with no regrets). There is also a larger sized similar bag from HELD available from SBS.