That sounds very strange. This lens has been out for a couple of years now, and it is almost unheard of for quality to go DOWN in a successful lens. I don't remember hearing problems like these...
One reason might be that the distance meter on the lens barrel isn't very accurate. If you set it to infinity when shooting things that are far away (which should be fine!) you are likely to have problems, because it's generally off. In fact, because the lens is so wide, when shooting something several meters away, the edges of the frame is likely to pick up things like the ground, walls, shrubbery (your feet!) that are much closer, and so may be out of focus. This is not because the lens is bad, however, just the user isn't use to this aspect of the lens. Just remember, the depth of focus on this is enormous! Unless you are trying to do close-ups of flowers or something, nothing should be out of focus (although field curvature may distort things near the edge). Just set the distance to much less than the distance of your subject. To repeat for the millionth time, if you set the distance at about 1.2 meters (4 ft.), at F5.6, everything from about 2.5 feet to INFINITY should be in focus. You can just leave it there and not worry anymore. I never worry about focus with this lens. For street shooting, I often don't even bother to look through the camera! I shoot from the hip, or just let my arm hang down and look off in another direction and click, click, click. People have no idea you are shooting them because you aren't focusing, and the camera may not be pointing anywhere NEAR them! It's so wide people almost next to the camera will still be in the picture. If you are shooting things close up indoors at F2.8, you will have to worry more about focus, but not too much more. Again, the DOF is giant on this thing.
Thanks for the input. Unfortunately, I'm fairly certain both copies I've had were decentered or otherwise optically sub-par. If you read reviews (particularly for the Rokinon variant) on Amazon or B&H, you'll see quite a few reports of severe copy variability. One of the top "helpful" reviews on Amazon states that they went through 3 copies before finding one that wasn't de-centered, but were ultimately very satisfied with the good copy they received. It seems that if you got a razor sharp copy on your first try, you should count yourself lucky.
I have played extensively with the focus ring, and have a good feel for how the actual focus deviates from the markings. The problem is that it seems the focal distance varies from one side of the lens to the other. For example, I can set the focus ring to the 3m mark, shoot a landscape, and the left 80% of the frame is sharp (except the corners at wide apertures). However, the right side is markedly softer. But interestingly, I found that if I included something much closer on the right edge (e.g., go out on my deck and shoot with the deck furniture near the right edge and faraway stuff to the left), the right side looks great! To me, this seems like a clear indication of decentering. If I crank the focus ring all the way to the end, past the infinity mark, the right edge improves a bit for distant objects -- but, of course, the rest of the frame suffers some. Seeing as I'll be using mine at infinity (for landscapes, sky, star trails, etc.) 95% of the time, it's unacceptable for me not to get even infinity focus across the frame.