My 42px80u came in today JUST as I was leaving work ... so of course I went straight back into my house and quickly threw the thing together :P Turned on the xbox 360, said "wow" then ran out the door. I was worried that the 42" would be too small, but I didn't have the budget for a 50" because I felt I would need 1080p. I was also worried about glare because I'm coming from a samsung 32" LCD. However, I'm actually happy with the size and the picture (even on a quick uncalibrated peek). Glare seemed to be no problem and the black levels were amazing compared to what I was used to on the LCD. I can't wait to get back and really set it up properly. I can't comment yet on whether I think I could use more than 720p, but I don't think I will. Prices are coming down now though, so if you find a deal where it's $100 difference, I'd probably just go for it for peace of mind (even though it seems like 80% of the people with an opinion say it wont make a difference).
EDIT: Be wary of comparing TVs in a showroom btw. There have been articles written about how places like Best Buy purposely use a lower resolution source, or screw up the picture settings on their low end TV's in order to convince you that you need the bigger better one with 1080p, 120hz, geek squad adjustment, monster cables, and whatever else they can sell. Also, splitting the source can apparently degrade the quality, so if one TV has it's own source, while another is split with 20 other TV's, that's not good. I would ask for them to setup a bluray player and have the same bluray playing on both TV's for you to make your judgment, rather than basing it on whatever they already have cycling on the screen.