Finding a mid-sized backpack that offers both a reasonable weight and compact size - without sacrificing functionality - really isn't that easy. A lot of models in the 30-45 litre range are either too flimsy, or don't have a decent harness - probably one of the most important things to look for in any pack, regardless of how much gear/weight you envisage loading it with.
Deuter's ACT Trail Lite 40+10 was initially used on a two-week trek in Nepal and provided a great compromise between weight, function, and most importantly, comfort when carried for four to six hours a day. The Trail Lite 40+10, as its name suggests, provides 40 litres of storage capacity, with an additional 10 litres available (via an extendable throat) if you're really keen to lug more gear, thanks to twin lid buckles that can cinch in, or adjust out, the pack's lid. The pack is perfectly suited to longer supported treks, but also a great option for a single-day/overnight adventure or alpine climbing.
Its beefy construction means you don't have to worry about dropping it onto rough ground, while its myriad features, including a stretch front panel (where I usually store an extra water bottle), ice-axe loops (used to hold trek poles when not in use) and mesh side pockets, have all copped heaps of use and worked perfectly each time.
The poles are held in with easily adjusted toggle straps (as well as the side compression straps), with the pole handle shoved down in the side pockets. The pack also has a water bladder sleeve, where I place my 3-litre Camelbak, secured via a small hook in the sleeve.
With any ungainly load, the compression straps have proved their worth, allowing me to cinch it all in tight to keep it from flopping around on my back when walking. The amount of adjustability and load securing is one of the pack's positives: no matter whether I've had a full or half-full load, the pack's myriad adjustments allow for a comfortable fit each time.
It is the Aircontact Lite harness system that stands out the most. The harness is a full-blown system, and includes stabiliser straps for fine tuning, Deuter's Vari-Quick system for back length adjustment, beefy hip fins for maximum load support, a nice strong internal frame (made from aluminium), and the surprisingly effective ventilation 'chamber' on the back (basically a channel that runs between two cushioned sections). Once I had it all adjusted to fit me, I have not experienced any aches or pains. Even though I was only carrying around 10kg maximum at any one time, even this reasonably light amount of weight can adversely affect you if your pack is not adjusted to fit your body.
The Deuter ACT Trail Lite 40+10's extensive feature list, excellent harness system and robust build, definitely qualify it for the '"bang for bucks" award.