First, it's a great handlebar bag, not unlike our old Nigel Smythe tweed handlebar bag.
Second, it's a stupendous saddlebag with a decent capacity between our Saddlesack Small and Xsmall. It can easily hold a pair of sneakers, six twelve-oz beverages, or a ulock.
Third it's a great pannier! It easily holds a sleeping bag. Use it in lieu of a stuffsack.
To save a couple of bucks we opted NOT to use waterproof zippers (like on rainpants), but in real world use it has not been a problem. If you're transporting valuable hand-written letters not written with archival ink and paper then perhaps using some kind of additional protection inside the bag is in order.
It's 12" x 8" x 3" with some room to stuff.
Here's a nice little story and video and more pictures about it's development.