Just went last week. Tried to go on our way up to yellowstone but didn't have change for a 20 for parking - bad planning. Decided to stop again on our way down, and thank goodness we did! We made it to the parking lot around 3pm on Friday. Not a ton of cars, maybe 20 or so. We saw a decent amount of people on the trail which was actually comforting because at points the trail splits off and leaves you wondering if you're going in the right direction. I certainly wouldn't describe it as crowded, at least not on the day we went.
The hike to the actual canyon was a bit longer than i had expected, it was probably around an hour before we actually reached it. However, that's mostly because for a decent part of it you're walking pretty slowly to make sure you don't slip on muddy trail or rocks as you walk through the water. Definitely wear hiking sandals or water shoes with decent soles and toe production for rocks. I wouldn't recommend tennis shoes as it helps to be able to feel the ground and rocks under your feet to keep from slipping or twisting an ankle. It's a pretty easy hike though, energy-wise. Few small hills but nothing that would leave you too out of breath.
Anyway, once we made it to the canyon, it was absolutely worth it. Gorgeous. The water was clear and nice and cold since it was probably 80-90 degrees out. The ladder constructed out of the tree trunk was helpful for getting over the first waterfall. Not too long after we passed the second waterfall we had to turn around because we still had an hour of driving left to do. It only took us about 45 minutes to get back to the parking lot as most of the walk back was downhill and I'm willing to bet 25% of our hike up was spent stopping to take photos.
All in all, this was the absolute best thing we stopped to do along our drive from LA to Yellowstone. If you can only make two stops in this area, do this hike and go to Zion. So great!