The first 8 miles from the highway are badly washboarded dirt road, with some deep sand thrown in for good measure. Then you drop into the Cottonwood/Marble wash itself for several miles, high clearance necessary, 4WD highly recommended though it's not technical, just slow going in spots. We didn't drive in all the way to the trailhead, not knowing the road situation up ahead when we saw a good spot to park up out of the wash we took it. Alas, that meant several extra miles of hiking; very scenic desert wash hiking, but we did run out of time and didn't get all the way to uppermost narrows in Marble Canyon itself (3 miles from the trailhead). Marble Canyon itself alternates between sections of narrows and sections of more open wash.
Next time, driving all the way to the trailhead would be my choice, if the backcountry office says the roads are passable. There is a good open area for camping up out of the wash about 1/4 mile before the junction of Cottonwood and Marble Canyons.
(Click on any of the pictures to enlarge)