Some characters might be critical. If they have a strong sense of the 'canon' Superman, they could nitpick at the details in the fanfic that don't match up. For instance, if the fanfic changes Superman's origin story too much, they might frown upon it.
Yes, the OPM manga has come to an end. The creators completed the narrative, providing closure to the fans. Although some might have hoped for more, what was presented was a comprehensive and conclusive tale.
Definitely! The OPM manga offers a lot of action, humor, and interesting storylines. The art style is also top-notch, which adds to the overall enjoyment. But, of course, it depends on your personal taste in manga genres.