We first saw someone sporting one of UD's Signature Series Vests at the Bear Chase Race this September, but the vest wasn't officially released until November. The AK and the SJ (the two more minimal models) sold out immediately. You can watch very detailed videos about all three of the Signature Series Vests here.
The most striking feature about these vests, at least to me, is that the bottles are carried in the front of the vest. When I first saw this design, I fell in love (specifically with the AK vest). I hate drinking the from a bladder and so I typically carry handheld bottles, which is tiring and cumbersome. After watching the video about the vest I thought that maybe my search for the perfect hydration system would be over. I'm very happy with the vest, but I am still searching.
Let's first address the reality of having bottles in the front. I now know what it feels like to be a 32DD and I thank God that I am not. In order to keep the bottles secure the vest needs to be strapped down very tightly. With two heavy sloshing things pressing into my chest I thought that maybe I was going to suffocate. As I drank more water, this feeling subsided but gave way to another problem. The sloshing becomes distractingly loud and if you drink from the bottles at differing rates then it becomes difficult to settle into that rhythmic groove you typically strive for on long runs.
I was pretty happy with the bottles themselves because the odd-looking nipple design allows you to keep the top popped open without any water splashing out of the bottle. The down side is that it is difficult to suck water out and you have to squeeze pretty hard to get a good flow. I get a sloshy stomach pretty often and I felt like more air bubbles got into each sip with this bottle's design. I think next time I will try it with some 10 oz bottles to see if that helps with the noise, but then that would be the same as carrying one 20 oz hand held.
Other aspects of this vest are fantastic. The fit is great with side adjustable straps. There are two sternum straps in the front that slide along a track. The pockets are adequately deep and have velcro to keep your goodies secure. You definitely won't be losing gels like I did with the MH Fluid vest in this race. I thought the storage capacity for this vest is perfect. You can easily fit a bladder and layers if desired but even if you don't have much in the back, the front load stays secure. As a fiber nerd, I especially like the cuben-fiber material that they used. Very breathable and no stretch. This bad boy will hold up for years to come.
Overall, I'd say Ultimate Direction did well with the AK vest. I don't think I'm going to be storing bottles in the front unless I was running terrain that required trekking poles. In that case it would be necessary to have hands free. But otherwise the storage capacity, fit, and fabric make this vest ideal for multi-hour running adventures.