If you could only have one pack...

This is the multipurpose pack to own. Other packs can be better for specific, narrowly defined uses, but if you want something that you can dial in to just about any activity, this is your pack. It's very lightweight and, better yet, configurable so you can adjust the weight to the situation. If you need a strong frame and a waist belt because you're hauling 30+ pounds, you're covered, if you need a frameless, beltless, loftless rucksack that only weighs a pound, you're also covered. On extended stay backcountry trips this pack will carry your ginormous wad of gear with relative ease. On day hikes with smaller loads it reconfigures superbly and doesn't feel excessively large at all. As an added benefit the pack can serve as an emergency 3/4 length bivy sack, and the frame pad can be unfolded for use as a bivy pad, or merely used as a complement to your existing pad on cold-weather trips (esp. when camping on snow). Similarly, on long alpine climbs you've already brought along your summit pack, just reconfigure in the field (takes a few minutes, no tools required) and you're good to go, or just take the loft plus the belt as a fanny pack.

One thing to note though, this pack is designed with the knowledgeable, experienced user in mind. For the most part it's pretty straightforward to use, but there are some complexities in configuration that take a couple minutes of effort to get the hang of. If you're a casual, blase hiker who prefers to remain ignorant of their equipment's finer points and just wants to cram stuff in a bag and start walking, this could cramp your style. But if you're a backpacker or climber who's used to dialing in their outdoor equipment systems, this will come as second nature.

And, of course, at this price, you just can't beat this pack. You could easily spend, say, $450 more for a Spectra Andinista to get something comparable.