In a post-Behind The Music world (man I miss that show), we all know the movements of the rise-and-fall-of-a-rock-group story. This makes it all the more important to imbue the story with something different, something distinct, or to tell it from a different perspective or a limited timeline. The Runaways almost has one of these things, in that it feels just as sleazy and filthy as the group itself. The tone feels completely right, and I don't want to discount that, because that's difficult to achieve. But that's also to say that The Runaways, as a band, are sleazy and filthy as show; beneath the surface they're an almost-decent group with the usual trappings of a band dynamic. Similarly, the film is an almost-decent one with the usual trappings of a rock n' roll picture.
So while it's difficult to point to its failures, beyond a sort of been there, done that vibe, it's much more difficult to point to its strengths, beyond the performances of Kristen Stewart, Dakota Fanning, and especially Michael Shannon. Stewart's exactly in her wheelhouse, imposing classic '50s method acting into the larger-than-life Joan Jett, while Fanning is either uncomfortable in her role, or playing a character uncomfortable in hers. Either way, the result fits. Shannon, meanwhile, is completely out of his mind, displaying the dedication one could only expect after his work over the last three or four years in a role only a step or two removed from Christopher Walken's in the Blue Oyster Cult sketch on Saturday Night Live (replace "cowbell" with "jailbait," and the two characters could hardly be more similar).
Its central failure, however, is in coming up with any kind of conclusion as to how The Runaways happened. According to the movie, one need only to assemble a halfway-talented group of performers, yell at them a lot, and bam, instant record deal. Stewart, as Jett, spends much of the movie trying to get the group to focus on the music, but what little music-writing we see is typically accomplished spur-of-the-moment, and completely formed. And whatever one thinks of The Runaways' music, that's just not how music happens. Moreover, whatever work they put in amounts to very little - there's nothing in this film that feels the joy of accomplishment the way we saw The Wonders react to hearing their song on the radio in That Thing You Do. Consequently, there's no great loss to the group disbanding near the end of the film. They never really seemed to be all that close (except, you know, for that one time), and they never really seemed to care all that much.