Have found that carefully considering what activities will be undertaken, a two pack combination fulfills all my needs (extended bushwalks generally in off-track thick bush situations, expedition trips, canyoning, caving, kayaking, etc):
Wilderness Equipment - Karijini (Large Expedition Model)
https://secure1.ilisys.com.au/summip/index.html
Summit Gear - an older model not currently in their stock, but placed somewhere between their Cockatoo and Lowry Day Packs (A large day sack)
https://secure1.ilisys.com.au/summip/index.html
These are both Aussie made = will wear very well in our conditions e.g. highly tear and abrasion resistant, streamlined so they don't get easily snagged on scrub.
The WE is one tough workhorse - the harness is the best I've ever experienced with it's choice of sizes, adjustments, etc.
(I've used Karrimor, MacPac and Bergheus - MacPac is very good too, though both Karrimor and Bergheus proved crap. The only thing stopping me from purchasing another MacPac was their replacement costs - had a harness fail after many years of heavy use but the sack was okay. After making enquiries into replacing the harness, it was cheaper to buy a new pack!)
I really don't understand peoples fascination with military packs - they're heavy and generally damn uncomfortable (but cheap). It's interesting to note that, according to Wiki, the US is moving towards civilian type pack design (MOLLE and MOLLE 2), with the Marines choosing off-the-shelf packs from a well known manufacturer. In civvie use, external framed packs (H and A-framed) went out with hobnailed boots!
Some observations:
1. A single sack with perhaps a top and rear pocket is the best as far as waterproofness goes. More entry points = more likelihood of problems.
2. Forget zips = can't be repaired in the field if they fail. Fastex buckles and drawstrings = no worries.
3. Side pockets are buggers as they inevitably get hooked up on foliage, get torn and loose stuff.
4. Never carry anything on the outside of a pack unless you want to lose it - if it can't fit in it, the pack is too small (or carrying too much crap)!
5. Use a pack liner (preferably a dry sac) to waterproof essentials like sleeping bag, spare clothes, etc.