It is true that in some ways it is a bleak film but in no way is it depressing. It's got a similar feel to Neil Labute's In The Company Of Men, ie very very funny, everything shot through with a healthy dose of black humour. The climax in the club and on the beach are some of the funniest and breathtaking scenes I've ever seen in a film. The dialogue is spot on. It did take a short while for me to get the measure of this but once I was in I was hooked. It's definitely worth sticking with. I haven't read the novel so I can't say how closely it sticks to Houellebecq's material but I have read his follow up Atomised and was pretty underwhelmed for the most part so maybe this is a good thing.
All in all see past the downbeat tone and get to the heart of a completely irrelevant man who is not afraid to show it.