I bought an Osprey 46 after shopping packs with good ventilation for the back, which my other packs did not have.
Wicking layers have nowhere to wick to with a pack that doesn't ventilate, and the Exos' Airscape suspension makes a huge difference. My first "test hike" was a 15 mile dayhike with heat index over 100 degrees, and it's probably the first time I've hiked in over 50 degree weather in Alabama that I didn't have sweat rolling down my back and between my butt cheeks. Our humidity often guarantees a lack of evaporative cooling, so I had doubts about whether the packbag being suspended away from the body would make much difference. It does.
The pack's first use was the second week of August, but it has ~200 miles on it already, due to weekly overnights and a trip that included 8 days of hiking.
You do not want this pack if it does not fit you perfectly(3 sizes, but no adjustable torso), or if you plan on carrying more than 30lbs on a regular basis(good support with a proper fit, but very little padding), but it has been wonderfully comfortable for me.