For what its worth, I own a Panny 500.
It is a great player and as Big Boss says its built to a really high standard. The picture is amazing, as is the sound through my Sony av amp. Motion is nigh on perfect too, much better than my old Sony 370 Blu Ray player.
I have two young boys and a big kids libary of Blu Rays amongst others. It does have the odd wobble with some of my discs. For example, Tim Burton's modern Alice In Wonderland - it freezes at a certain point on the film and omits a high frequency noise / buzz. It has also done this on Drive recently. The firmware is up to date and the discs are immaculate. I must point out that my Sony 370 had wobbles with certain discs too, I think its just the nature of complicated Blu Ray players and most are prone to the odd strop. Sometimes the picture quality (fed to a Panny GT30 tv) just makes me grin with joy (the last two Batman movies were beautiful to watch from a picture perspective).
The Sony was not out when I bought it, the 500 was the boss at its price point. I still have the Sony 370 upstairs in the bedroom.
I have not had a chance to demo the 790 yet but from what I have read it is very very good. Had it been out to choose from when I bought the Panny I probably would have gone for that but I am still a very happy chappy.
For me, the Sony menu system is so much more user friendly. As also mentioned it's a SACD player if that was a selling point. Plus, I hate the touch / swipe remote on the Panasonic. Some like it but I cant get on with it - its gimmicky and not very intituitive.
I also cant seem to get the 500 to talk to / stream to my Sony Vaio laptop - it can be done but I have given up (have Airport Express anyway) - far too complicated for my simple mind.
It's a difficult choice though still. I have a Sony av amp and yet a Panasonic tv (which I notice you have too, probably a broken one!). My Sony amp has some hdmi switching issues with the Blu Ray player (keeps switching to tv input and not hdmi1 input by default, if that makes sense) which can be a little annoying too.
I think if I had a choice now I would go for the Sony but I am sure their are plenty of 500 owners that will gladly tell you that I am wrong.
Hopefully that helps