My Jam2 just came in the mail on Tuesday, and I got my first chance to practice pack it last night. Because I had no idea how to go about packing it, I went with the "barrel method" in that I wrapped my Therm a Rest self inflating pad and filled the center with my gear. Now, I still have my stupid heavy Eureka tent, but even with that, I was very happy with how the pack held up using the sleeping pad.
I do have a bit of a problem with the shoulder straps not exactly hitting my shoulder blades where I want to, but also it was my first time packing it up with all the gear I typically take with me. I'll probably be happier with it the more I use it.
Overall, though, I love the little guy and am very happy with the purchase. The barrel method seems to work well, but as I plan to replace the ground pad in the next year, I'll have to play with it more.