After I presented at the San Francisco REI, I received an email from someone who said that they will use the Gossamer Gear Whisper Backpack on the JMT. I replied:
The Gossamer Gear Whisper will be good for the JMT, although it's a bit extreme for most people. The Gossamer Gear G5 might be better if you need to carry more than 4 days of food (or warm clothes). I used the G5 and Maiu used the Whisper. I will use the G5 on the CDT. The G5 is just a little heavier than the Whisper, but has more capacity. Most folks find it hard to squeeze everything they need into a Whisper.
Resupply along the JMT is inconvenient. That is why we walked 240 miles without a resupply from Tuolumne Meadows to Kennedy Meadows.
If you do the JMT in the Aug or Sept, then it should be easy with both packs. It just depends on your capacity needs and how often you resupply. Consider not shipping food on the JMT, because it is expensive to receive a package. It is cheaper to just buy food at Reds Meadow and Muir Trail Ranch or go in the hiker box for food.
Lastly, most JMTers would prefer a backpack with a bit more structure (and weight), like the Gossamer Gear Mariposa Plus. I use it whenever I have to carry a lot of gear/food for glacial travel, for example.
For even more features (and weight), consider GoLite's backpacks.
Finally, MountainLaurelDesigns.com makes some ultra light packs that are excellent.