I shake like Michael J. Fox when he's off his meds (ow! ok! ok! bad me!) and I find that a monopod helps quite a lot when a tripod is not practical.
Think sport shooting or shooting events where you're in a crowd - setting up a tripod is NOT going to be possible at times. A monopod, on the other hand - just set it to a little less than eye-level (or whatever height you feel most stable / comfortable), and you can very easily move around. Pick it up, see a shooting opportunity, set the pod down, and you're ready to shoot - no unfolding of legs or making sure it's level.
Downsides? Well, it can kill creativity. Can, not does. Since you have the convenience of your camera and lens supported by the pod, you might not think to detach the camera in order to get a perspective shot that's not at normal human eye-level. It's possible - unlatch the quick-connect, or rotate the head 90 degrees to get the pod out of the way, but it's a bit of a bother. Shooting things in flight over your head might be problematic.
But if you're shooting things from eye-level, and want good mobility AND stability, IMO a monopod is irreplacable.
-Keith (who hopes MJ Fox has a thick skin
)