I've got a Large ALICE pack. For kicks, I took it while walking the dog today. After 5 minutes, it was digging into my sacrum/pelvis... mind you, the pads are actually new. It was uncomfortable, but didn't get much worse over the 40 minutes after that. The problem seems to be that the waist pad is only about 16" long, so it doesn't wrap around your hips, only sits against your back. Also, the shoulder straps didn't put much weight on my shoulders, being mainly holding the pack against me.

On the plus side, the external frame did a GREAT job of keeping my back from sweating (the 35 degree weather didn't hurt).

I'm debating throwing all my stuff into a CTD MOLLE bag, but various posts here have said it's not that durable. Also, it's a weak internal frame, so it may be a lot hotter than the ALICE.
http://www.cheaperthandirt.com/MOLLE125-41518-1964.html