We're a gear weight conscious family and it's the reason we picked the Osprey. It's the lightest of the high-capacity packs. On my home scale it weighed in at 3 lbs 6 oz without the included rain guard (which weights another 5 oz).
A few complaints about this overall strong pack:
1. The storage compartment for the rain guard is in a bad spot (exterior zipper on the bottom of the pack). If you use the included straps to attach a pad under the pack, you can't get the rain guard out. They should have just left that off and saved the weight of the zipper and extra layer of fabric and suggested you put the rain guard in the upper compartment.
2. Most of the straps are too long. The hip belt is big enough for my 50" waist for pete's sake! I suspect they were going for lots of sizing flexibility, which it has, but it was overkill and adds unneeded weight and frankly on my son the tail of the hip belt hangs down so low it could be a safety issue if left to hang. (I'll be trimming about 8 straps to get it down a few ounces)
3. The attachment point for the sleeping bag barrier (the fabric the separates the sleeping back portion of the main compartment from the upper half) is too high on the front (the side up against your back) of the pack. The effect is that the hydration sleeve too compressed by the sleeping bag to slide a platypus bladder in after the sleeping bag is loaded. Not a huge deal except when you want to refill mid-day.
But overall, those are minor complaints and it is a good solid pack and better than either the REI or Gregory equivalent pack.