Storage
The Mountainsmith Apex 80 does not incorporate the traditional backpack design and is the only pack in this category that does not contain the traditional lid, storm flap design.  This did not hamper the storage of the pack, since it contains a zippered front panel pocket, with a seam-taped zipper, that takes the place of a lid pocket. Two large zippered U shaped access points provide access to the interior of the pack, much like a suitcase.

The Apex comes with two stretch hip-belt pockets that provide good storage, but a couple items came close to falling out during some scrambling; zippers would have improved the utility of these pockets. Two side stretch mesh pockets are very deep and easily accepted 1-liter bottles, allowing easy access on the fly.

Hip Belt
The Apex incorporates thick air mesh foam in the hip-belt, which wraps well around the hips and provides very good comfort. The hip belt also provides good lumbar support that works well in transferring the load onto the hips and lumbar area for increased comfort.

Backpanel/Shoulder Straps
There is a thick padding on the shoulder straps and an easily adjustable sternum strap.  These work together to make the pack very comfortable with weight up to 50-55 pounds.  The back panel provided good airflow, but not quite as good as some other Mountainsmith packs.

Stability
The Apex 80 is very stable carrying loads in the 50 to 55 pound range, in a variety of terrain and on the occasional scramble. It did not float or sway and was very comfortable carrying big loads on steep ascents or descents.

Overall Comfort
The pack is very comfortable and did a good job hugging the body. The shoulder straps and wraparound hip belt made carrying loads from 50 to 55 pounds quite easy. The stiff frame added to the stability of the pack and made it comfortable on long treks with heavy loads.

Durability
The pack proved durable in a variety of weather and tough terrain conditions. During some rugged and tough backpacking trips that included a large amount of bushwhacking, it did sustain a small tear in one of the side stretch mess pockets and a very small pinhole in the fabric on top of the lid. While the conditions were very tough, I would not have expected these issues given the relative newness of the pack.