Got one of the handlebar bags. It comes in two parts, a "sleeve" for carrying it on the bike, and the inner bag which is like a small kit-bag. It attaches to the sleeve by Velcro. You can mount it horizontally if you need all the capacity, but if not carrying much, you can fold up the kit bag and mount it vertically so it doesn't stick out either side.
If you have a traditional sprung saddle like a Brooks, the fixings will attach from the loops or saddle springs, so it will double up as a saddlebag. Very useful for what they cost.