I just got the 42px75U a few days ago. My current tv prior to that was the 37PX50U from 2 years ago. My wife let me "upgrade" to a new TV ever since my young daughter started watching cartoons and she would hug up the only TV we had. I figured rather than buy a small flat planel TV which costs almost a grand, I'd spring for a few dollars more and get myself a bigger tv.
Here's the differences I've noticed in the past few days, and the tv is not fully broken in. 1)false contouring is less pronounced on the 75U than on the 50U. 2) the 75U does a MUCH better job with 720p, the 50U does a terrible job, cutting the resolution by half. 3)black levels are about the same to me. 4) 1080i signals appear to be the same, although the look jaggier on the 75U, this could be due to the size difference, I'm not sure. 4)pixel structure is more visible on the 50U when sitting about 3-4 feet away, this could be due to the different pixel structure between the 37" and the 42".
I'll post some pictures soon.