At this point, Gunn would have to drop the ball in a catastrophic way to fumble this film. The box office and general audience is one thing, but JG is putting his entire weight behind a classic Superman story with character studies that will follow closely to the average joes interpretation of what makes these characters beloved. There is no deconstructing going on this time around.
For fans of Superman? I just don't see how this will fall flat for us, unless the writing and story completely fall apart (say what you want about Gunn, those very things are his strengths).
I am starting to teeter towards Superman being a big hit, especially opening weekend. It feels like a sleeping giant in-waiting. But it's still so incredibly hard to put a finger on of what Superman can bring in. Pre-sales will be highly interesting.