The Bottom Line: For technical uses like long days hiking, climbing, mountaineering or skiing, give these a miss. The are likely a great pant for survivalism, photography, and hunting.
I used these pants on trips to the Bugaboos hiking and rock climbing as well as -5-degree Celsius mountaineering adventures around the Bow Valley. I also used them for cragging. I have added the experiences of my friend, who uses them ski touring and ice climbing.
I got the Fjallraven Gaiter Trouser to solve a problem I had. Mountaineering required heavier duty, very water resistant pants. But the approaches were often hot, making such pants very uncomfortable.
The Keb Gaiter is a variation on the iconic Keb pant, Fjallraven's flagship product famed for its heavy duty G-1000 fabric and wax water resistance, except with zip off gaiters that transform the pants into shorts. It's basically a heavy duty version of dad zip-off pants.
The fit is surprisingly svelte and hugs the body tight. Despite the gusseted crotch and stretch material between the legs, these pants always felt constrictive. High leg movements were always held back when rock climbing, and it was noticeable when resistance kicked in even while hiking. I found the shorts too long, and the oddly heavy duty zippers would chaff against my upper calf as they weren't sufficiently covered by a flap. The pockets are all really tight. I could hardly cram an iPhone 5 with case into the big cargo pocket. This does ensure a streamlined look, but it looses points for lack of capacity. The seat did not rise all the way to the small of the back; instead, a heavy seam sat midway up the backside. The result is a constant sensation that your pants are falling down.
On my trip to Abbott Pass in late season we got a bunch of wet snow. Expecting this, I had applied a couple of coatings of wax as directed. Within minutes the fabric had wetted out, even after three coats of wax. The pants are also stuffy and hot. Even in shorts mode with the rather immodest side zipper open, they still managed to be less comfortable than my heavy duty softshell pants.
Now, my buddy swears by them. He uses them ice climbing, snowshoeing, and even ski touring, and of course hiking and scrambling. Perhaps the fit is better for him. And this is an exceptionally popular pant, so I may be way off base here.
I will say this about the pant. They look great. They also are perfect for people who don't have to go far but maybe need extra durability. I can see them thriving as a hunting and bushwacking pant, especially in the fall. I can absolutely see why the survivalist crowd loves them because they are durable and great for kneeling, or otherwise mucking about on the ground.