A few points about the 42PJ5
1) Out of the box, the image quality is TERRIBLE. I stuck in some Blu-ray and HD DVD discs and it was awful. One of the worst I've seen out of the box. It was stuck on Vivid or something and contrast was high enough to blow out all details. Some of the settings have an Edge Enhancement "feature" too. What the…?
However, after a quick and dirty adjustment partially just using my eye and using the LG Wizard feature, it's much better now. However, I will need to break out the Digital Video Essentials disc to do a proper calibration. The contrast and detail is OK now, but the colours need some tweaking. Fortunately, there are a lot of adjustment options, and more than with other TVs. Plus, it's independent for each input as far as I can tell.
Hopefully the calibration from my HD DVD Digital Video Essentials will translate over to the other HDMI inputs OK. The cool thing is that if you use a built-in video mode setting it lists that setting name, but you can still modify it, and it will display it as the name with "(User)" after it if modified. Very informative. There are two "Expert" setting options too, which allow you to change lots and lots of stuff. It almost feels like a service menu.
2) It's a mirror. Last night it was fine, because I generally don't keep a lot of lights on when watching TV. This morning however, the glare from the windows was very prominent. That said, while my previous matte-screen LCD didn't have the glare, it was still difficult to watch during the day, just not quite as bad.
3) I didn't perceive any input lag on direct DTV standard def channels (bypassing the cable box). I only thought I noticed something when I was dealing with HD over HDMI but like I said it seems that turning off all processing corrects any perceived lag. I don't have any real way of measuring it though. I've just been using Blu-ray and HD DVD titles. eg. Hammering scene at the end of Batman Begins.