I have the Deuter Futura Zero SL 40 for overnights. Just used it once and had a bit of an issue with it being bouncy. Like my center of gravity was off a bit.
I have the Deuter AC Lite 25 daypack and love it. I do think it makes packing harder. Also the Deuters fall over if you try to sit them on the ground...not sure about the others.
I do think it helps with ventilation on all but the most humid trips. I notice a big difference in spring and fall but only a bit in the middle of summer.
Unlike the Osprey's the Deuters have an open mesh bottom and you can't put your bladder in there. A couple of people I hike with have the Osprey's and put the water bladder in the mesh in cooler seasons to get more room in the pack.
Millette J
Yes, the bladder must go behind the trampoline. Otherwise it's impossible to remove and replace without unloading the pack, not to mention it reduces space. I used it hiking out of the Grand Canyon in August and didn't have any issues with the water getting uncomfortably warm.
I think overall, the trampoline is a great idea, especially in warm climates.