all LG displays have image retention issue (or feature per Apple). Samsung might have more serious quality issue that's why it was discontinued since Oct 2012. Or maybe due to their furious legal fights.
You can find more info out of 451 pages at: https://discussions.apple.com/thread/4034848?start=6750&tstart=0
I contacted Apple Care within return period and they refused to replace the screen with a Samsung-made. Since there is at least one-year warranty, I'll monitor its severity and try to replace within warranty if worsens.