All of which might go some way towards explaining why the Valerie Solanas biog has taken as long as it has to reach the screen, despite being undoubtedly rich pickings. For Solanas may only be tangentially famed through the lives she skirted - Warhol and The Factory crowd, Maurice Girodias at the Olympia Press - but from such scant pickings director Mary Harron and co-writer Daniel Minahan have fashioned a rich, funny, rewarding little movie.

Harron, who worked previously for The South Bank Show and the BBC's The Late Show, originally envisaged a documentary which, in the fullness of time, became docu-drama before blossoming into this full-scale feature. A hyperactive, splenetic Lili Taylor is Valerie, proto-feminist agitator behind the Society for Cutting Up Men and author of the infamous S.C.U.M. Manifesto. It would be easy to play her for caricature, but to her credit Taylor takes the part in all seriousness. Valerie's approach was scattershot, from prostitution to hustling passers-by for cash in return for a dirty word - what word, asks one; 'Men,' she replies - to her ill-disciplined but spirited writing. Taylor registers humanity behind the confusion, pathos even, in her dealings with the greasy Girodias (a terrific Lotharire Bluteau) or her sincere confidence in the never-quite-of-this-earth Warhol when she takes her coprophiliac play, Up Your Ass, to him to produce. It's a powerful showing.

The film, although ostensibly sympathetic to Valerie, is surprisingly commensurate in its depiction of the eponymous Pop Artist. The real man, so far as we know, was a blank canvas - a mirror even - onto which followers could project, and as Warhol, Jared Harris (son of Richard) is perfect, capturing that same disinterested fascination we see in interviews. As Valerie becomes convinced that he has stolen her play, he is never less than courteous, even when she turns up at his office that fateful last time with a gun. Harris nails the voice - and the man's frightful complexion - perfectly.

A bio-pic of Warhol himself would be difficult going on impossible, being cruelly reductive of a man who, in a sense, was the people he surrounded himself with. Harron's film (like Schanbel's forthcoming Basquiat, wherein David Bowie reprises the role) effectively does the job by proxy: Craig Chester is Warhol's agent, the obsequious Fred Hughes; Donovan Leitch assistant Gerard Malanga; and there is a spectacular - and oddly affecting - performance from Hollywood pretty-boy Stephen Dorff as transvestite superstar Candy Darling. US indies Yo La Tengo make a show-stopping appearance mid-movie at a lovingly recreated Factory party, as house band The Velvet Underground.

Musically the film is extremely rich, pushing a variety of periods - Chris Montez, The Lovin' Spoonful, Love - and contemporary - R.E.M., Luna, and Holland's Bettie Serveert - recordings as well as a potent string-driven score by ex-Velvet John Cale.

Harron's direction is busy and sharply edited, owing much to Gus Van Sant's My Own Private Idaho in its willingness to leap between the formal and impressionistic. Thus, for example, we get plainly realised interrogation scenes rubbing up against the thrilling quick-cutting of Valerie's introduction to guns, itself choreographed to The MC5's call to arms, Kick Out The Jams. It's the details - the words pouring out of Valerie's typewriter; the books and newspapers with which she surrounds herself; the policeman at the hospital carrying Warhol's iconic silver wig between two fingers - that reach deeper into this illusive world.

In the end, maybe, the film is not quite analytical enough to really puncture the enigmas that were Solanas and Warhol. Then again, there can be few harder to decode than this pair - one who remained tantalisingly superficial through effectively recreating himself as nothing; and the other who history itself has largely chosen to forget. When she is asked to explain her actions by the police, Valerie tells them she isn't ready; when she was, no one was listening. The film skilfully suggests that there is more to be said, while remaining provocative, stimulating and unexpectedly entertaining viewing.