I use the G4 and like it (although it's getting to be a bit too big for my overnights and weekend trips). The pack has always moved well with me once it was properly adjusted, and the silnylon fabric is tougher than I was anticipating (although I'm not ready to drag it behind my car yet).

As far as weight...some have posted that frameless packs are effective up to 30-35 pounds of weight, while others set that limit a little lower. I've carried up to 25 pounds in mine, and it worked, but wasn't comfy by the end of the day. But then again, I'm a wussy girly-man