People shouldn't approach this as a superhero flick (or you'll be terribly disappointed as there is hardly any crimefighting involved), nor as a film about mental illness (because the characters are 'sane'). Rather, it is a film about two people living in their own little world. One in which the superherotheme merely serves as a defense mechanism against the cold-hearted society that has damaged them so much.
I'm sick and tired of superhero films. Even the new narrative that superheroes are damaged sociopaths doesn't make the films more interesting for me. On the other hand, Griff's portrait of the superhero as manifestation of mental illness manages to be mostly refreshing. The final act (the "heartwarming" bit) falls flat - too stale and predictable.