I've got two handlebar bags.
One is a Brooks Millbrook, the other is an Ortlieb. They are about the same size.
I love them both. The Brooks has many more miles, but is wearing fine. The Ortlieb is the more versitle of the two. It locks to the handlebar (keeps the bag from being stolen, not its contents), and is waterproof (I've never had water get in the Brooks either for that matter). The Ortlieb is much easer to attach and remove, its quick release system is very well designed.