Its not a true toggle. Press shift once to start running and you keep running as long as you continue to press forward (w). Press shift again or hold it down and it doesn't do anything. You just keep running. Stop pressing forward and you stop and go back to the default of walking.
So shift isn't so much a toggle as it is a "running starter". The only way to drop out of running is to stop moving forward for a millisecond. If you are already running then just release (w) and then quickly press it again and you will be walking.
So far I like it. I think I like it better than I would having to press shift again to walk. I definitely like it better than having to hold down shift to maintain a run as it is in A16.
I believe I meet all of Aldranon's requirements to comment and opine about this as well....
I give the movement scheme an A.