Day-hiked this on a weekday at 5pm and saw 2 other groups, not near as crowded as I thought it might be. It is a great hike for beginners, lots of different stuff to see. The campground at the trailhead is pretty nice too. I was a little confused finding the trailhead after I parked as there are no signs for "Staircase Rapids loop" but plenty of marked signs for other trails. Just follow the sign for "Rapids Bridge" and you'll get there. The trail isn't 100% of a loop, it starts and ends at opposite sides of the parking lot about 0.2 miles apart. The first half (if you start by walking UP from the parking lot) is a basic walk through the woods for a mile to the bridge. It's uphill the whole way, but you'll only gain about 300ft elevation. There is one pretty cool dry creek crossing with a nice wooden bridge, thanks park service! At the halfway point is the mini Tacoma Narrows suspension bridge. That area is very nice and has alot of little paths branching out for exploring.
The second half is much more scenic, as you walk downhill along the Skokomish River pretty much the whole way. Again, lots of side paths venturing down to the river for better views. Near the end keep an eye out for a sign labeled "Big Cedar". You can't see it going the direction I was on the loop, but was lucky to be given a heads up by another group. I would have totally missed it! It is a very short trail (maybe .1 mile) and leads to an old fallen tree. It's the biggest tree I've ever seen. If you see the bee that chased me all the way back to the main trail, slap him in the face for me please.
My GPS said closer to 2 miles than the 4 miles listed on WTA, but who knows. Bugs weren't too bad, on the drive up along Lake Cushman the horseflies were swarming but not biting. Driving up to the trailhead, Lake Cushman has some excellent views, and lots of pull off points on the road.
All in all, excellent hike for getting a feel of the Olympics. If you go, consider camping and possibly doing a bigger trail while your there as well!