The only leaks I've experience have been my own fault and not the K4. One of my K4's I bought from ProVape and it came with the P3 base, which I had to install. I didn't tighten the a9 screw as tight as it should have been, but that's fixed now. I've since found that by taking the K4 completely apart and lubing all of the o-rings with PG, and putting a light coat of Chapstik on the a4 and a7 seals, the Juice Control turns as smooth as butter.
Looking at the exploded parts diagram you'd think that it's a very complicated device. Well in a way it is, but once you get a grasp on how all the pieces work together it's a very elegant design. I'd be willing to bet that those who are having issues with the Juice Control are using clones, but they just don't want to admit it. Because if they are using an authentic K4, why are they posting about it in a forum and not talking to the people at SvoeMesto?