A sheer 215m drop, Jim Jim is awesome after rain (when it can only be seen from the air), but its waters shrink to a trickle by about June.
The falls are reached via a 4WD-only track that turns south off the Kakadu Hwy between the Nourlangie and Cooinda turn-offs; they're about 56km from the turn-off (the last 1km on foot). The track is open in the Dry only and can still be closed into late May; it's off limits to most rental vehicles (check the fine print).
A couple of tour companies make trips here in the Dry. There's camping nearby at the Garnamarr Camping Ground.