The action scenes alone make Spider-Man 3 good in my books. It's one of the only superhero movies that makes me think "yes, if these superheroes were real this is exactly how they would use their powers in a fight."
I don't think Spider-Man 3 is completely without merit at all. One of the better things about it are the action scenes. There are like a handful of moments of just wince-inducing hard hits.