Very well constructed, with one very painful exception
First off, as others have noted, Osprey packs are very well constructed. The pack walls are constructed of a very durable nylon that exhibits at least a reasonable degree of water resistance (despite not actually being waterproof), so light-duty tears don't seem to be a potential problem. There are plenty of compression straps and options for strapping and rigging gear to the exterior of the pack (which is, for my part, every bit as important, if not more so, than the interior capacity of the pack). The shoulder straps and hip belt are solid and provide a snug fit, which is a pre-requisite of any good pack, since conserving energy on the trail is contingent upon being able to limit unnecessary side-to-side movement of the pack while hiking. The AirScape backpanel does a really good job of allowing cool air to circulate in between you and the pack to prevent heat and sweat buildup, and this has a lot to do with the ridged design of the backpanel, itself, as well as the nylon mesh that overlays it. This last feature, however, is the fatal deal-breaker.
The aforementioned mesh is constructed of a very coarsely woven, stiff nylon that is absolutely unforgiving on accidentally exposed skin, especially skin that directly overlays the generally bony parts of the lower lumbar and upper sacral regions of the back. Making matters worse, the lower lumbar portion of the foam backpanel is actually convex toward the body, which is to say, curved into the back, which reduces the effective surface area of the pack's contact with the lower back, thereby focusing more pressure on an even smaller area. As a significant portion of the weight of a pack should be distributed across the midsection and not the shoulders, the convex backpanel and its coarse mesh outer result in a sensation not unlike that of a sanding block being rhythmically and methodically ground into the small of your back with every step you take. Unless you're wearing, say, a heavy fleece or other thick top (tucked in under a belt), you're going to have the blood, blisters, and resulting abrasion scar to prove it. Hiking uphill is especially painful since more of the weight of the pack is focused on the small of the back, and also because aggressive inclined hiking frequently tends to hike the back of your shirt up, thereby exposing the skin to even more torture. Your mileage may vary depending on body type, but for some, I suspect that summer hiking with naught but a t-shirt is going to be a no-no unless you've specifically packed light (which rather contravenes the purpose of buying a 5,200-cubic inch pack) or are prepared to administer regular shots of Toradol into your sacrum.
In short, this was the first pack that, in a relatively short (4-day) medium laden (45 pound) trek, made me second-guess how much I liked backpacking. Some pain and discomfort is always to be expected when backpacking -- your gear is simply designed to limit, not prevent, discomfort -- but this was excessive. I've read similar complaints about shoulder chaffing caused by the Aether 85's shoulder straps (which are covered in a similar, though less coarse, mesh), and I really think Osprey could address the problem by choosing different materials. In the end, you'll need to try the pack for yourself (as you need to do with any piece of gear), but be wary of potential problem areas. Backcountry has an extremely commendable "no-questions-asked" return policy, so you've got nothing to lose if you're theoretically sold on this pack. I have to send this one back, however, and specifically for the sake of my own.