Got a really late start so didn't get to Islip Saddle until about 2:00 p.m. Was expecting heat, but it was actually pretty dang nice. Ascended the PCT on Williamson's southeast side to the junction with the use trail, then on to the summit. The trail is well worn and easy to follow with scenic views opening up to the AV to the north. Too bad it isn't longer, although I suspect you could extend the trip considerably by following PV Ridge west toward Pallet. Lewis was just "myeh" IMO.
I've decided I really like the peaks in this area. Islip, Williamson, Waterman all super nice. Worth doing again and again.
Some pics:

Original West Fork Ranger's Station now at Chilao

Over the shoulder look at Islip

Looking westish out toward Waterman and the Twins

Views of the high desert opening up.

Trail View

View east toward Dawson Saddle and Lewis

View south down Bear Creek Canyon I suppose

Obvious junction with the use trail to the summit (go right here)

View west just below summit

Summit cairn. Didn't see a register.

Summit view of the high desert

Trail companion on the return trip. I was impressed with this little guy's aggressiveness.

Summit register on Lewis.

Looking back to Williamson from the summit of Lewis

View toward Baden-Powell and Burnham from Lewis
The end. Yay mountains!