After borrowing an older Gregory Iris pack to head up Mt. Whitney and finding it to be very comfortable, I opted to purchase my own Gregory pack. This pack needed no break-in period. Wore it right away on a 14 mile hike, then a 20-mile hike - no sore shoulders, no rubbing, no problems at all. Carried 5 liters of water to start on one hike and once the pack was on, I could barely feel the weight. The back stays reasonably well ventilated, and the various pockets and pouches are all useful, particularly the ones on the hip belt. 32 oz Nalgene bottles fit fine in the side water bottle pockets and can easily be removed and replaced without taking off the pack. For years I've preferred to wear lumbar packs rather than a full day pack on shorter hikes, but this pack is comfortable enough that I am opting to use it on more of my hikes.