My previous pack was a Kelty Super Tioga, an external frame pack which carries almost 6000 ci of crap while weighing 8 lbs! So the atmos was quite a change being internal, ultralight (at only 3 lbs, i have only seen a few packs lighter, and none with a frame), and fairly small (3000 ci).
My atmos is to small for me, I really should've gotten a large, however, it still works fine, and with loads around 20 lbs, it is quite comfortable. However, when loaded at around 40 lbs, it hurts my back pretty significantly, this is far over its designed, and even specified weight capacity though, so I don't really see this as much of a problem.
For those of you coming from older, bigger (and external frame?) packs this pack may seem to small, that was my concern anyway. I've found it quite capable of holding everything for a 3-4 day backpack trip though, and have used it snow camping, and on an assent of Mt. Shasta, so its got enough room to bring winter clothes along as well.
I haven't yet been rained on while using it, so I can't decide if I need a raincover, but it would appear to be waterproof (or at least highly water resistant.
The best part about the little pack though is the suspension (frame). It is 2 stainless steel bars running vertically which transfer the weight from the shoulders to the hips quite well while keeping the pack under the 3 lbs. A side benefit to this is they also hold the back of the pack away from your back, so its only a superlight mesh touchoing your back, making the pack nearly as breathable as an external frame. The hip belt looks as though it'd be super uncomfortable coming from an external frame, however in reality it works amazingly well for loads under about 30 lbs, and it breathes! Because of this (and the shoulder straps), the atmos actually breathes better then my external frame pack!