I really like the Osprey backpacks. I like the design, construction, everything especially for what I might ever do which may not ever be that much. My 'excuse' is that my son may some day do Philmont and they need backpacks and it recommends 75L min for internal packs. and I really only want to go internal pack.
Someone recently told me that the 70L would be more than adequate to fit my needs, but for only $20 difference I could have a larger pack but everyone says "Don't get too large of a pack or you will just pack too much in there" But I also hear that food, gear, etc. can take up a large amount of space...
I don't see myself ever being an 'ultra light' backpacker, I don't plan on cutting off tags or straps to save miniscule ozs on my pack. I did however buy an UL tent, Sleeping bag, and pad, as well as water purification system, and other things needed for backpacking, with the last thing being needed is the backpack!! :-)
I checked the Osprey site and they say the 70 is good for 3-7 day trips and the 85 is good for 5-10 day trips. REI's web site shows 2-5 nights needing 60-80L packs and 5+ days needing 80+ size packs. Seems a bit confusing.