I haven't yet played the demo, waiting for the xbox360-version, but from the videos, it looks like Konami recreated PES 5/6 using the engine of PES 2011.
I don't know if I will be happy with that, cause I had hoped Konami would take PES to the next level instead of recreating the classics.
I guess it's like Seabass said, it's a game for the fans, and the majority of fans wished a game that would recapture the magic of PES 5/6 within this current-gen-engine.
Tomorrow I will see if it makes me happy, too.
It's definitely not PES5/6 recreated. Which is a shame in a way.