I have two Ortlieb Ultimate handelbar bags, and matching mounts that I leave on four of my bikes. I've found that once you get used to the convenience of a handlebar bag, you rarely want to ride without one (off-road MTB is probably the exception).

@spinnaker, not sure what you are thinking of when you say "Obviously you have never mounted an Ortlieb bag. They use a cable and not screws."

I have two different types of Ortlieb mounts, and both use screws as well as the cable.

For the OP, if you want a handlebar bag *mount* that is truly easy to take on and off, find an old-style Cannondale bar bag (or a knock-off version). These had a steel frame that simply looped over the handlebars and stem, and then the bag was kept from bouncing around by two straps that connected to the fork dropouts. Worked fine for all the years I used one, just not as sexy as the click-on/off style.