Probably an often asked question but here goes anyway.
I'm in the market for a new load carrier. Requirements are as follows
Hold 55 - 70ltrs worth of kit
Be comfortable enough for weekend trips and weeklong trips that are self supported
Be robust enough to eventually deal with things such as The Cape Wrath Trail
Not weigh a ton!
Be fairly reasonable price wise
I won't be doing any Alpine ascents in it and will mainly be doing 3 season Hill Walking in the likes of the Brecons and the Lake District.
I have looked at the North Face Terra 60 but was thinking of getting people's opinions before I part with my hard earned. I have been tempted by the GoLite series of packs but the look just a bit too flimsy for my liking, would the Osprey series be a more durable but still lightweight option?
I have the 70ltr and its an awesome comfortable pack that goes with me in winter time when more/heavier kit is needed.Great weight too at 1.70 for a 70 ltr,and very robust and well built.
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Also try as many as you can on to see what fits best as others will say no doubt...
The crux ak57: weighs 1.5kg approx, probably the most durable rucksack you can buy, very comfortable carry, simple design, practically waterproof, i have yet to discover water ingress, i used it for a month long trip carrying around 14kg which it dealt with easily. It could probably carry double that weight with no problem.Price around 150 which is a lot but this pack will last you years
The osprey exos 58: weighs between 1.15 and 1.2kg (depending on what size you require), loads of features, stow on the go system for walking poles, hipbelt pockets, numerous pockets elsewhere so plenty of storage options. very comfortable carry. Not as durable as the crux or as water resistant and the hipbelt especially can soak up a fair amount of water. price around 140 which again is expensive but both these packs are seriously good qualit
Go somewhere with a good selection and try on as many as you can that tick your various requirements and then get whichever is the most comfortable with a representative weight inside. For the "represenatative weight" you can either take the kit you'd expect to carry or, simpler, borrow rough equivalemts from the shop and use those. This procedure also clues you up on how easy they are toget stuff in and out.
Different folk are different shapes, and different harnesses work better or worse on those different shapes. I really like the old Lowe Parallux harness on my Kangtega, but a friend after a loan of a pack found it hideous. OTOH, she found my Macpac Ascent to be the most comfortable pack she'd ever worn. You just can't tell without trying, and once you get into bigger sizes there are usually bigger differences in the carry because the hip-belt plays a much bigger role in taking the load.
I wouldn't be put off the GoLite packs by the fact they look flimsy. They're made from Dyneema, which is just about the toughest pack material around and will stand a lot of abuse. I've got a couple of OMM packs also made from Dyneema and they've both taken a lot of hammer and look none the worse for it. Also, the OMM Jirishanca and Villain are both "light" packs made from Dyneema, yet are popular with climbers who aren't known for their gentle treatment of gear. As Peter says, comfort is the key -- but why carry a heavy pack if you can find a light GoLite or OMM one that fits you well.
After annoying the staff at every outdoor shop in Covent garden 3 evenings on the trot I settled on the Osprey aether 60, I'm not sure how much it weighs but it is not significant. The Compression System is awsome and I frequently use it as a day pack because it is so comfortable.
Try a few packs out, I prefer the more verticle alpine styling of the Aether than my mates golite which looks like an old fashioned bergen to my mind, She also ends up with a lot of kit strapped to the outside, only the tent poles live outside of mine along with walking poles if I am not using them.
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The Didster..... 
