It's fun. A standard three-ball cascade juggle fits pretty neatly into your regular arm swing, so it actually feels pretty natural.
I watched a guy do a five-ball joggle, for an entire marathon. That blows my mind. (One of the things you discover doing endurance sports: no matter how crazy you are, there's somebody out there crazier than you.)
Definitely use beanbags rather than something that rolls. Bending over to pick up dropped balls is hard enough without having to chase them further.