I did this very popular hike in early August. When I set out from the Lodgepole visitor center, I didn't encounter anybody till I crossed the bridge (about 1/4 mile). But I was surprised to find the number of people along the trail.
This is an out-and-back hike, 1.7 miles each way (not a loop as the other reviewer says). It follows the river for much of the course, allowing ample opportunities to dip your feet. It is pretty level most of the way, although you'll need to tackle a short rocky stretch towards the end.
The scenery is pretty good, although not spectacular. There's plenty of shade along the way, but the end of the trail, however, is totally exposed. And there's not much room for a lot of people at the end, so you'll need to climb some boulders up or down to enjoy the scenery.
The waterfall was just barely more than a trickle at this time of the year. But I've seen pictures of the waterfall during spring and early summer, and it'll be totally worth the hike then. So I wouldn't go out of my way to do it any other time of the year, in which case my priority would be Sherman Tree, Moro Rock, Crystal Cave, and then Tokopah Falls.
This review is the subjective opinion of a TripAdvisor member and not of TripAdvisor LLC.