The 507 isn't without annoyances - if you buy the optional speakers (a bit of a pet peeve on mine about the 507, since the 427 comes with speakers) and use the Component input with an Xbox 360, you need to use one of the SCARTs for the audio.
It's cost-cutting like that that is frustrating.
Since a number of installers were annoyed (as were the clients) that they done away with the media box in the current gen, then I doubt Pioneer will be making the same mistakes this time round.
You're right about early adopter nervousness, but none of the top makers make a bad panel, and I would imagine you've nothing to worry about.
As for the extended warranty - if you're not paying for it, get it. Otherwise, use the Sale Of Goods Act as that's what it's there for.