I've carried my 58 on some Appalachian Trail and other mountain hikes this summer, a couple weeks worth, in the hot, rainy, muggy Mid-Atlantic area. I haven't really thought of it as being too hot at all, to me it's not bad enough to complain about. One day I had it on for 7 or 8 hours without taking it off. The last day of my last hike my shirt was totally soaked at the end, but the front was nearly as wet as the back. I'll take the extra heat over a pack that holds the weight away from my back though, the Osprey AG packs don't want to change direction with me when I make a sharp turn, and they don't support their listed max weight in my experience. If the AG worked for me I would be using it though, just for the extra ventilation, and the out-the -front water bottle pocket on each side as opposed to just one side on my 58.