I have a Mariposa Plus and have carried it with up to 30 lbs - the comfort zone is around 25 lbs and it has to be really well balanced to be comfortable above that (at least for me). With the new stay it works very well and is more comfy with a bear canister than before. I have light but bulky gear (some of it anyway) and the capacity is pretty good - light stuff I use during the day goes in the mesh pockets, the rest inside.
One of the things about these packs - if you order and try around home, it's likely they will let you return it if it doesn't work. If you do use it on trail and it doesn't work out, there's a pretty good secondhand demand for ultralight gear, especially over at the Backpacking Light forum.
"In the beginner's mind there are many possibilities. In the expert's mind there are few." Shunryu Suzukihttp://hikeandbackpack.com