For me it's the same with characters from Friends. I can't see them in another role (except Jennifer, it's kind of odd to see her in other things but not as much as the others).

That being said, George is an absolutely great character and one of my favourites in sitcoms :) Just reading these comments makes me want to rewatch Seinfeld.

I would also recommend Curb Your Enthusiasm, but there is a difference: Larry David is neurotic, but even in his fictional Curb Your Enthusiasm persona he is extremely wealthy and successful. That is a big difference from George Constanza, who combined being neurotic with constantly being adrift employment-wise and has little to recommend himself.