I'm not quite sure why Bitter Moon is so poorly regarded. For me it's one of Polanski's best – certainly not as good as Chinatown and Rosemary's Baby, but far better than The Pianist, Knife in the Water and The Tenant.

Maybe it gets swept under the carpet because of the lurid sexual content. Or then again it could be the melodrama that puts people off. Or it could even be the fact that it features Hugh Grant. But for me all those things work wonderfully for the film – the sex gives it an honesty (there's no pussyfooting about in this film), the melodrama shows the absurdity of relationships and Hugh Grant… Well, Hugh Grant just makes the film funny, albeit not always intentionally. (He's much funnier here in a 'straight' role than he ever was in the risible Four Weddings and a Funeral.)

But even though I personally love the film, I guess it's quite easy to get the wrong end of the stick. After all, the film begins with a ridiculously stiff middle class English couple on a cruise liner (Hugh Grant and Kristin Scott Thomas). You can't help but wonder if it's a joke – they seem to have wandered in from a Merchant/Ivory production. And then you have Oscar (Peter Coyote), a disabled gargoyle of man who proceeds to tell Nigel (Grant) the story of his doomed romance with Mimi (Emmanuelle Seigner). At the start it's all sweetness and light, with bad clichés thrown at you from all directions. And it's at this point that you either get Bitter Moon or you don't. If you take the film completely seriously you'll probably think it awful, but if you realise that Polanski, with the English couple (Grant wears a blazer and slacks for fuck's sake) and the sickeningly cute courtship, is having fun, then you just might have a grand old time yourself.

Just to further illustrate how obvious it is that Polanksi's having a laugh, take the first flashback. We get a slow pan from the camera, a fade and then some hilariously unsubtle 'French' music begins playing. Polanski's tongue is rooted firmly in his cheek. And then in his narration Oscar spouts bad cliché after bad cliché. But rather than annoy, this works superbly for the character – Oscar wants to be a writer; however it's painfully obvious why he's never been published. A sample of his voiceover: "Nothing ever surpassed the rapture of that first awakening. I might have been Adam with the taste of apple fresh in my mouth. I was looking at all the beauty in the world embodied in a single female form and I knew, with sudden blinding certainty, this was it!" And then after that we get a scene where Oscar rubs Mimi's feet on a park bench and another where Oscar tells Mimi he loves her as they fly in circles on a fairground ride. This seems to be your standard Hollywood romance on steroids pumped full of saccharine.

But just when you're getting nauseous, Polanski throws in a comic sex scene. Here, in the scene in question, we see Oscar and Mimi sitting at the breakfast table. At first they just look at one another, but then Mimi dribbles milk all over her breasts. And then after Oscar's licked it off, she administers a blow job. And to cap the scene, the toast pops up out of the toaster as Oscar comes. Now how can anyone not enjoy a film with a scene like that in it?

But although the breakfast scene is amusing, it also marks the point where the film begins to darken. With the initial thrill gone, Oscar and Mimi have to resort to sex toys and sex games to get their kicks. And although that works for a while, Oscar soon tires of Mimi – she bores him.

The scenes that deal with the breakdown of the relationship are probably the best. They capture the way relationships can imprison people and the way people can quickly tire of one another. And after Oscar's initial infatuation it's quietly shocking to witness how unfeeling he is towards Mimi – he barely reacts when she gets on her knees and begs him not to leave her. He really doesn't care anymore.

But just when you think this is bad, the film takes things even further. To try and get rid of Mimi, Oscar resolves to make her life a living hell. He treats her badly, embarrasses her at a party, and worst of all, abandons her just as they're about to go on holiday. But what makes it shocking is that it's actually kind of funny. I'm sure most people have had someone they want to get rid of but can't. Therefore it's actually kind of fun to live vicariously through Oscar. As he says himself, everyone has a cruel streak.

And it's fun watching Nigel's reactions to these stories. He tries to pretend that he's listening out of politeness, but despite his protestations, he loves every moment. He's a consummate voyeur. And it was a twisted stroke of genius to have Grant play the part. He's a poor actor, but there's a lot of fun to be had in his reactions – his response to Oscar's golden shower story had me howling, as did his reaction to Oscar talking about sucking on Mimi's clitoris ("Steady on, old boy"). Oh, and Grant's dancing in the party scene at the end has to be seen to be believed.

But as amusing as the film is, there is a serious core to it. It accurately captures the selfishness and cruelty that is present in many relationships (although, of course, the film takes it to an absolute extreme). And plus, in certain sections, it's actually quite sad. There's a wonderful little scene where Oscar and Mimi ride a fairground wheel together. The music is beautiful, the photography is dazzling and Mimi tells Oscar how happy she is. It's simple moments like this that make relationships worthwhile. Sure sex is of vital importance, but it's the togetherness that generates lasting happiness. And you feel that Oscar has lost sight of this. His pleasures seem rooted entirely in the physical aspect of their relationship. And it's telling that it's after this brief moment of contentment that Oscar dumps Mimi. She no longer has anything to offer him - he's taken all that he can from her (well, or so you think).

And when you think about it, it's quite startling how Oscar debases Mimi. Here's a woman who just wants to be loved and he uses that love to further his own strange appetite. And he abuses her so much that, in the end, she becomes exactly like him - they're mirror images. So maybe it's a belated act of compassion that Oscar shoots her and himself.

But of all the things I love in the film, it's the New Year's Party scene that I love the most. The moment where Fiona (Scott Thomas) and Mimi dance to 'Slave to Love' (got to love a bit of the Ferryman) is one of those wonderful bits of cinema that takes you completely by surprise. When I first saw the film I was as shocked as Nigel. But the scene has a strange kind of beauty to it. And I don't say that because it features two attractive women kissing. No, you kind of feel that Nigel deserves this. He hasn't cheated on Fiona by the strictest definition of the word, but he wants to. And so the dance and the kiss is both revenge and a kind of wake-up call. You feel that, much like Oscar, Nigel is losing himself. And so perhaps by being crushed under Fiona and Mimi's high heels he realises what sort of emotional devastation he was trying to inflict upon his wife.

But you could also argue that Oscar's murder-suicide is a bizarre act of compassion for Nigel and Fiona, as well. Having lead them so far along his dark, twisted path, he's maybe unconsciously telling them to wake up, that the answers for the problems in their relationship lie within, that you shouldn't look for answers in other people.