I returned a 13" MBA with an LG screen, because it was dimmer than my wife's Samsung, and the contrast seemed a bit off. It just didn't seem to have the level of contrast that my wife's has. Thing is, I didn't really notice it until I was working on installing ML on her computer. That's when I realized that at least to me, there was a difference. I'm not saying that it was an LG vs. Samsung thing. It could have just been the screen that I had. I also noticed that the viewing angle wasn't as good. The screen had a pinkish tint when viewed from below. I know, not a big deal and nobody views it from that angle, and the pink was because of my color profile. But I could not adjust the angle of the screen so that it would get rid of the greyish gradient at the bottom of the screen, without the top of the screen being a bit pink. I did install ML on it, and I have to say that I didn't like the profile that came with ML. It was way too blue. I did calibrate it with my Spyder, and got much better colors with it. But in the end, it was just too dim, and it lacked contrast. So back it went.
I waited a week, and I've ordered another one. It should be here on Monday. Hopefully with ML on it. If it's an LG, I'm not going to return it just because of the screen manufacturer. As long as the track pad is good (I've had two with screwed up trackpads) and the colors, brightness, and contrast are good, and there are no dead pixels, it's a keeper.