There are few films about poets and fewer that are based on poems. So Rob Epstein and Jeffrey Friedman's Howl deserves credit for going beyond biography and attempting to showcase Allen Ginsberg's 1955 poem of the same name.

Howl was the self-consciously Whitmanesque, sexually explicit, book-length work of protest and celebration that launched Ginsberg's long career as a poet, and more important as a symbol of the so-called "Beat Generation" (though he thought the term was silly).

Famously or infamously, it laments the way the 29-year-old poet "saw the best minds of my generation destroyed by madness" and exalts sexual encounters with bikers and sailors.

The film was originally intended to be a documentary, but has ended up a strange, often fascinating hybrid: part re-enactment, part-courtroom drama, and part animation, with Oscar nominee James Franco as Ginsberg.

It often feels like an effort to replace the brand-image of Allen Ginsberg, the grey-bearded, impish hippie icon, with the equally superficial image of Ginsberg as a young handsome gay icon - the earnest writer looking fetching in a white T-shirt as he taps away behind a pair of Arthur Miller spectacles. The music, the black and white scenes of Ginsberg declaiming to an enthusiastic hipster audience, the clothes, all tap into today's fascination with the retro Mad Men glamour of the 1950s.

As for the animation that the filmmakers employ to illustrate the poem, there are moments when they capture something of Ginsberg's feverish aesthetic. More often they are wince-inducing.

Every word in the film is taken either from interviews or the transcripts of the court case in which Ginsberg's publisher was unsuccessfully prosecuted for obscenity. The result can be dramatically inert, especially the courtroom scenes. This is despite the presence of Bob Balaban as the judge, David Strathairn as the prosecutor, Jon Hamm as the defence attorney, and, playing expert witnesses, a host of famous actors like Treat Williams and Mary Louise Parker.

The filmmakers quote too much from Hamm's dull, anti-censorship defence lawyer, apparently blind to the fact that he was a literal-minded fellow whose efforts had little impact on the verdict. It is as if the directors' besotted worship of Ginsberg and everything he stood for has crippled their intellectual and aesthetic judgement.

Franco does a fine job of capturing Ginsberg's intonation though he has a much richer and deeper voice, and is of course better looking. But his Ginsberg sometimes gives off an aura of smugness - and it is not clear if this is intentional or one of the actor's own tics.

It may well be the latter. After all, the filmmakers, as true believers in the Ginsberg myth, do not seem to have noticed that their own work, especially the courtroom scenes, ironically provides evidence of the tolerance of the evil system against which Ginsberg was rebelling, and that some of his rebellions, like his support for paedophilia, were less than admirable.

On the other hand, the film works as a showcase for the poem, which is far more effective performed out loud than on the page. Howl may seem mannered today, but it does contain genuinely enjoyable imagery, and captures a sensibility that must have seemed amazingly daring and exciting in the middle 1950s.

Unfortunately once you have heard large chunks of Howl again and again - as you do in the film - it is hard not conclude that what Ginsberg, Kerouac and co needed, but resisted more than anything, was editing.

By the time the end titles arrive, one phrase from the poem comes to mind, a phrase that could stand for the whole Beat ethos with its adolescent worship of "coolness", its romanticisation of druggy self-destruction and its cult of self-indulgent writing: "endless balls".