I've done plenty of backpacking from generic external frames from my youth, a much coveted Mountainsmith from my college days, Large ALICE and even a High Speed Gear Inc. modified Large ALICE during my Infantry days (and still being used), a little experimentation with Arc'Teryx, Gregory, Lowe and a couple of top-end REI brands and now, Kifaru. I own several Kifaru packs, with their EMR being the largest. As most already mentioned, you really need to figure out how much you plan to be carrying and what type of activities. Some of the light-weight packs are great for the trail, but if you're going to be packing into the back of vehicles, having it tossed around, carrying off-trail where it's much more rugged, you'll need a tougher pack that won't rip or blow out on you.
I know Kifaru's packs are overbuilt and weigh more than typical packs when empty, but their suspension is one of the best I've used. I've carried our Army ALICE rucks for several years...they plain suck, especially with anything over 40 pounds. I've done several "road marches" with my Kifaru EMR loaded over 100 pounds, and the suspension/waist belt makes it much more comfortable than carrying 60 pounds in my modifed large ALICE. I know there are other packs in the same category of Kifaru (Arc'Teryx is one), but my experience with Kifaru both from a civilian backpacking perspective and military use has been exceptional. Never had one fail on me yet, and some of my mil packs (Zulu and Pointman) have been overly abused outside of hiking or roadmarching with them (stepped on, sat on, thrown in and out of helo's and fixed wing aircraft, crammed in the back of trucks, pulled, pushed, smashed...). I can easily give Kifaru a thumbs up on a dependable, robust, flexible, and comfortable pack system.
ROCK6