Here's a datapoint --
I just bought a 15" BTO, it came with an LG screen, and it definitely is dimmer than I feel comfortable with.
At max brightness, it is much worse than my several-years-old 17" MBP with the TN screen. Interestingly, it is even dimmer than the 2012 13" rMBPs we have here (though to my knowledge, LG manufactures all those screens, too?) The dimness may be the manufacturers way of trying to prevent image retention. I have no idea.
In any event, it is going back to Apple, as the LED will only get dimmer with time, and I don't think having to squint is a reasonable tradeoff in a $3000 computer.
Hopefully some folks at Apple have taken a close look at the new IGZO screens (the Dell XPS 15 is incredible-looking, but the thought of running Windows 8.1 is stomach-turning.)
They don't dim as quickly as CCFL, but in my experience they still dim over time. LG manufactures the vast majority of IPS panels. It actually surprised me to see Samsung come up with the rmbps.