I've had a couple of Kelty packs, with my 'latest' being a Red Cloud from about 2002. It sees about 80 miles of trail each spring/summer, and shows virtually no sign of wear.
Honestly, I wish it would because I'd love an excuse to buy a new, lighter pack.
They are nearly bullet-proof, but that comes at a weight cost. The canvas on my older model is very, very sturdy. It's been treated a couple of times with water-repellant spray, and is lightly lined with another coating that is beginning to peel a little.
The zippers are pretty robust, but could be smoother - newer models seem to use a different zipper tooth, but I can't verify that.
The padding is still holding up to my 55-65 lb loads, with minimal plastic deformation. The down side is that unless the pads fit you nearly from the start, you will always hot spots (I do). Easy to manage with moleskin or blended fabric clothing.
The hip belt is fairly stiff, so distributes weight pretty well, though if you do cinch it down tight for scrambling, it can dig into you and cause soreness over time. That also increases fatigue, but fortunately you can adjust most elements on the fly to shift weight.
To be fair, I don't beat my packs up - a few scree falls, and short drops, but nothing like sliding down 50' of granite or ice.
They're solid, well-performing packs. I'm pretty easy on packs, so my only complaint is weight.
Well, and I like to buy new things once in a while, and this damn thing won't give me an excuse.