As beautiful as this area is, I wouldn't recommend this trail for any but the most surefooted, with plenty of rubber left in their knees. Af the 2.8 miles to Vesper Lake, I doubt there is a 1/4 of actual walking-on-dirt trail. The rest is loose rock or tree roots. Speaking as someone with 45-year old knees, the worst part is down.
I left the trailhead at 9:40, and made it to Headlee pass in 2.5 hrs., just like it says in the 100 hikes book. I didn't try to kill myself, and got lapped by 3 other solo hikers. Starts with an up and down traverse to the Stillaguamish river, then a bridgeless crossing (go up above where the trail meets the river), then a 1/2 mile climb up an avalanche clearing, around a ridge of Morningstar Peak to enter the main valley above a cliff. Up, up, up - and super big steps most of the time. Beautiful Doug firs growing in trios. The trail goes up the main valley through small trees with some Salmonberry (not ripe yet). Up across rokslides, switchbacking through trees, then entering the only way out. The slot gully was "the worst I've ever seen it" according to a climber I met later on the Vesper Peak slabs. To me it just seemed like more of the same. Amazing rock strata - almost vertical, and lots of conglomerates. I think.
After Headless Pass, another big rockslide below Sperry Peak, and across boulders to Vesper Creek. After a side trip to Vesper Lake, I decided to give the climb a go, and set up the ridge at the stream crossing. If you aren't climbing the peak, the meadow above the campsites makes a really good spot for lunch - views into the cirque, and south into the upper Sultan River. The ridge trail goes up really steeply through trees, and then breaks out to a small flat spot. After this the fun begins up beautiful polished slabs of white granite - still super steep but easier to take.
You can't really get lost, and there's a boot path up the ridge if you can make the saddle. The little streams have the best handholds, I noticed. At the top of Vesper Peak, a cloud blocked the view south and west, and other peaks had their tops hidden, but there were still amazing sights to the NE. The north face is straight down. Cooper Lake and the smooth white granite ridge between Vesper and Big 4 are amazing.
The way down was so arduous, however, that I don't think I'll be back anytime soon. Too bad because this is amazingly close to Seattle and so so pretty.
Surprisingly, mousquitos were biting nearly the whole time.