tl;dr Great vest, great price.
I love this vest and paid full price when it originally came out, so at this price it's a smokin' deal. I run trail races from 50K to 50 miles in length and have used this vest many times to get me from aid station to aid station in a race or on a solo trail run. I've used hydration vests in the past from Camelbak, Nathan, and Ultraspire. The last few years I've preferred the vests offered by Ultimate Direction.
One of the times I used the AK Mountain 3.0 was on a self supported race in Alaska in 2017 (Resurrection Pass 50 miler). To give you an idea of its capacity, I carried: a 2 liter hydration bladder, a 500 ml. water bottle, water filter (Sawyer Squeeze), bear spray, bug spray, rain coat, long sleeve tech shirt, small bivy sack, camera, phone, about 2000 cal. of dry electrolyte drink mix (Tailwind), & food in ziploc bags (potato chips, fritos, bean burritos, dried mangoes, nuts, chia seeds). The various pockets and pack were filled pretty close to capacity, but the vest was rock solid. I was very pleased with its performance.
There are 2 vests from UD that are comparable to this model: the Jurek FKT vest & the Mountain Vest 4.0 (Mountain Vest 4.0 is the update to this vest-they dropped the AK-Anton Krupicka, from the title). Since Massdrop occasionally sells the Jurek FKT, I'll compare the AK Mountain 3.0 to the Jurek FKT. (I wrote a similar note on the discussion page for the FKT vest.)
1st off, I prefer the front pockets better on this vest. I can carry more stuff in the front with the Mountain 3.0 vs. the FKT, which means easier access to more items on the go. Another difference is the bottles/pockets sit a little higher on this vest vs. the FKT. (I don't really have a preference either way, just something to note). I do prefer the Collapsible/Flexible bottles that come w/ this vest to the Hard/Flat bottles that come w/ the Jurek FKT. I don't like the Hard Bottles & swapped them out for the Flexible Bottles that came w/ the Mountain. (The great thing about the flexible bottles is you can collapse them as you drink so you don't have to listen to your liquids sloshing around as you run. This may not be as much of an issue for hiking). On this vest, the material that sits against your body seems to breathe a little better than what is used on the FKT.
Another difference: the 2 front straps on this vest are fixed to a rail on the front of the vest that allows you to slide them up and down to adjust for fit and comfort on the fly vs. the FKT's 2 front straps that are locked in place into 10 possible slots on each side of the front of the vest. Both vest's front straps can be tightened or loosened instantly as needed but to move a strap higher or lower on FKT vest you'll need to take the pack off and insert the end of the strap into a slot either above or below where it was. As a practical matter, I don't find myself adjusting the straps up or down that often. Only if my load has varied significantly from the last time I wore the vest. Also note: I haven't had any problems on the Mountain 3.0 with the front straps loosening up on the rail & sliding up/down on their own.
On the FKT, I like the "burrito pocket" on the upper left chest as a place to put your phone. It's a nice, water resistant pocket. Easy to access w/ your right hand.
The FKT vest is more of an update to the SJ 1.0/2.0. vest. The update to the SJ 3.0 vest is the UD Ultra Vest 4.0.
These are the biggest differences I noticed between the AK Mountain 3.0 and the Jurek FKT. They're both great vests, I just prefer the Mountain 3.0 to the FKT vest. If I was grading them, I'd give this vest an 8/10 and the FKT a 7/10.
Disclosure: I do have the new Mountain 4.0 vest and use it instead of the 3.0, for my trail races. It's got a few upgrades to this vest, but it also comes at 2 1/2 times the price of this vest. This is a great price for an amazing vest and I've recommended it numerous times to new ultrarunners.