May be this is the wrong answer but I had to ride 3 miles to work on a spare bike last winter with a rucksack and had a sopping wet back in seconds. I would always avoid anything on my back
I don't have quite such a sweaty back, and the Osprey has a massive spiel about the clever technology it has implemented to reduce soggy backage, but if it's hot, you still get a bit of a soggy back. As Heltor said, though, if it is your thing the Osprey quality is top notch.