Fit/Comfort
For being one of the largest packs I tested in this roundup, the Osprey Rev 12 also rides amazingly well and bounce-free. It’s part of Osprey’s relatively new Rev line of trail-running packs, which include 1.5L, 6L, 12L, 18L and 24L options. With a nod to Osprey’s heritage as a successful backpacking company, the Rev 12 is unusual in the trail-running world for coming equipped with a hip belt and more substantial mesh harness system to promote breathability between your back and the pack. Incredibly, these features do not feel like overkill on the pack; rather, they help it ride exceptionally smoothly, even when fully loaded with gear.

Carrying Capacity
The pockets on this pack are exceptional. The main rear compartment is like a small daypack—sizable enough to stash most anything you could ever want on a trail run. A small zippered top pocket keeps a few essentials separate, organized and at the ready. Meanwhile, there is ample storage along the front straps and sides of the pack—some zippered, some open with elastic mesh, all of which can keep an astounding number of items secure and easily accessible on the go. The rear bungee is excellently designed for stashing a couple extra layers.

Function
One particularly innovative design piece is the “DigiFlip” media pocket—a detachable smartphone-specific pocket along the front strap. It’s sizable enough for small- to mid-sized smartphones without bulky cases—and a decent place to stash your phone, as long as you don’t intend to also use it to take photos on your run. Kudos to Osprey for their creativity in designing the pocket, though I might prefer a more generic pocket with versatility for either a phone or something else more useful to have up front, like an additional water-bottle pocket.

Hydration System
The included 2.5-liter hydration reservoir has some nifty features to it—including a fantastic bite valve on the tube, a “shelf” system to help the reservoir slide easily into the pack and sit steady while running, and a detachable hydration tube for easy removal and refilling. The reservoir retains its shape, even when empty, and offers a welcome buffer between your pack and potentially awkwardly shaped items in the rear storage compartment. Finally, the circular lid on the reservoir didn’t always track correctly. It seemed like it was sealed but once running, water leaked out and soaked the inside of the pack.

Versatility
The best part of this pack is that it can, no doubt, double as a day-hiking pack, mountain-biking pack, day-to-day urban pack or even run-commuting pack without necessarily looking like a running hydration pack. (The Rev 24L is an ideal fastpacking option for those who want to run multi-day, self-supported routes with ultralight gear.) It’s likely overkill for most well-supported trail races or shorter training runs, but for longer pushes in the high alpine where weather can shift rapidly and layering systems are key, it’s hard to go wrong with this stalwart of a pack.