I used this bag to bicycle across the US. It is terrific. The attachment is secure, it is completely waterproof, & you can expel all the air from the bag.
I highly recommend it & would not buy any other bag.
I use the Apidura Handlebar Pack on my commute every day. I like that I can put it on any bike, it has a tight little shape that fits between my handlebar curls and stays put when i slam into potholes. It's easy to take on and off and the accessory bag clips on in a very well thought out way. I added a shoulder strap and that worked out great for walking around town. It fits some tools, a chain lock, a change of clothes and some other junk that I carry around with me. Sometimes I take off my jacket or my gloves and tuck them in the crisscross bungees mid ride when I get warmed up, I really love that feature. The accessory pack is a great addition for my wallet, phone, and some snacks.
I purchased this bag for a three day credit card bike trip, where I was also using a seatpost pack for larger items. The bag worked well for smaller items (threw some sandals in there and some extra repair kit stuff, and a paperback book). On a road bike, the space is limited by the distance between your hoods, and that was a bit of a pain to deal with. But the bag didn't move and the bungee in front was great for a light and to hold a lightweight jacket on the outside of the pack.