I had the same decision to make over the holidays, and after weeks of looking, I chose the 5080 and have not regretted it for one second. I cannot stress this enough: the picture on the Panasonic--and I viewed it in the store, at at a friend's house, etc.--is just not as vibrant as the Kuro. The 5080 has anti-glare, but not the matte screen of the 50px77 and it makes a big difference--and that's before you get to the deeper blacks. The 50px77 really looks dull to me in comparison, and even my (2007) Panasonic-owning friends concur.
And yes, it's cheaper, but it depends on where and when you buy. I got the web price and a price match from a later sale on my 5080 and it turned out to be cheaper than my original plan.
I'm very excited to see Panny's 2008 lineup, but I would keep the above in mind if you're considering the 50px77. It's a fine tv and may be the best in your budget, but as others have suggested here, I would keep shopping around.