I got started from the Southwest Ridge trailhead (the Williamson Rock side) at a little after 2:00 PM, and settled into a relaxed pace, stopping occasionally to snap a shot. The trail is in excellent condition except for a couple rocky/rooty spots. After leaving the PCT, it's steep and a bit less traveled but very easy to follow. This route is one among a few in the San Gabriels that remind me greatly of the Sierra Nevada: 1) Williamson, 2) Twin Peaks, 3) Pine Mountain.
Also, there is a very interesting mix of forest here. I spotted the following on the hike up (some from a distance): Live Oak, Big Cone Douglas Fir, White Fir, Ponderosa Pine, Alder, Big Leaf Maple, Incense Cedar. It's interesting to see the douglas fir and white fir mix between 7,000'-8,000'. Also, for some glorious reason the manzanita far outnumbers the buckthorn!
Summit views from Williamson are somewhat unique in the range. The absolute prominence is 800' (from Williamson-Pallette Saddle, ~7,600'), and there are huge canyons on three sides, and long sweeping ridges rising from many angles. It makes for dramatic sunset viewing:
View Northeast: Valyermo is the green-ish valley midground; Saddleback Butte can be seen right of center
Summit of Mt. Williamson, with moon, and summit register
Mt. Baden-Powell (R, taller) and Mt. Lewis (L, shorter)
Typical up-down semi-scramble near the summit, with a mix of white fir and douglas fir
Just after sunset, Twin Peaks just right of center, Mt. Waterman at right
EDIT: correct prominence after looking at map, d'oh!