This is a great loop with some challenging moments due to blowdown and sun exposure. I started and finished this backpack at the Pole Creek Trailhead. The Pole Creek area was completely burned out in 2012. It's not the most fantastic part of the hike, but it's nice to see that it's slowly recovering. It also had a ton of blowdown on the return from Matthieu Lake, which was very tedious after a bit. Many trail reports I read said to skip this part. I'll leave that for you to decide. The rest of the loop was beautiful. The major highlights were Green Lakes, Moraine Lake, Linton Meadows, The Obsidian Limited Entry Area/Opie Dilldock Pass, Matthieu Lake, the views of all of the mountains, the wildflower displays, and of course, the people that I met along the way. The low lights were as aforementioned, the burned-out, Pole Creek area, and the sun exposure. It was in the 80s most of the time I was on the trail. When I wasn't in shade, the sun made it feel much hotter. On that note, make sure you have extra water for the Obsidian Limited Entry Area/Opie Dilldock Pass area and from Matthieu Lake to the Pole Creek trailhead. You'll certainly need it. Obsidian Falls is a great water refill spot, but you'll also pass another creek (White Branch?) which is your last option until Matthieu Lake. At Matthieu Lake you can refill for the rest of your trek through the Pole Creek burn area, which has no water in the summer months.
PERMITS: As of this year you need a permit to overnight. The permit system certainly made for a much quieter experience in the wilderness. I'd go hours without seeing someone. The busiest section was definitely the PCT.