I have tried a few ultralight packs along the way and though I liked the idea of ultralight packs I never found them very comfortable. To echo Slo-Go'en, the UL packs I have tried were basically sacks with straps, light as hell but I could not get comfortable in the shoulder straps and without ventilation back sweat was an issue and caused other problems including chafing. That said, I probably had near the limit of about 20-lbs or so that I stuffed into them and had to repack them a few times to get the pack to ride better and get some padding against my back. The overall experience was educational and through that I figured out that pack chassis and suspension meant more to me in comfort than the 2-pounds weight savings and financial impact.
So, I went after the weight issue differently through examination of gear I carried which proved to do the trick and left me with a comfortable pack and load. Every time I went out for any overnight trips I kept a journal of what I used. I was surprised at all the small and seemingly necessary stuff I carried that was rarely, if ever used and started getting rid of things that weren't routinely used. I would pose questions to myself, Did I really need two pairs of spare socks, or did one pair work? Can I upgrade rain gear so the jacket can double as a windbreaker when needed? Should I get a canister stove as opposed to my faithful Whisper Light stove? Suffice to say, through some little used gear removal I saved about half a pound, followed by another three pounds with some needed gear upgrades that served dual purpose or provided better gear at less weight.
The end result was a total weight savings of nearly 3-pounds to base weight. I was fortunate for the opportunity to try out a few ultralight packs. I think they are a great idea if one is comfortable wearing them. It was a good lesson to learn and I continue to this day with the "Did I Use It" Journal.