C3PX said:

I guess when it comes to fan edits, I have really high standards. If it doesn't look good enough to be able to fool someone who has never seen the film before into believing it could have been the studio release of the film, then it isn't up to my standards. Which is why I have never bothered to release any of my own edits.

I agree with you here. Frankly, there's not many that reach that high standard.

There are only two that I've seen so far that I feel are truly "seamless" in the way you describe:

Terminator 3: The Coming Storm by Uncanny Antman
A.I.: Super-Toys Last All Summer Long by JasonN (this one isn't 100% seamless, but it's close, and it dramatically improves the film; the ending is slightly jarring, but it's much, much better than the theatrical version's ending)