I couldn't agree more with Nomad 7's and I A HVR's comments. A perfect laid back Sunday morning movie. The humor is subtle (exact opposite of "slapstick" as one misguided commenter noted).
But what always ceases to amaze me is how often I find myself wanting to come back to this movie over and over. I originally copied this movie onto VHS about 12 years ago when it was premiered on one of those Pay Cable free weekend previews(HBO maybe?). Had never heard of it previously. Don't know why it wasn't marketed that well. ?? When DVD's were released en mass, it was one of the first movies I replaced. A great combination of cast and writing. Plus, the back drop of Montana wilderness doesn't hurt things either (beautiful).
It's probably not the type of comedy for everyone, but what is? If Adam Sandler type stuff is up your alley, this probably won't be your cup of tea. This movie needs your full attention. The humor is mostly in the dialog.
I believe my next viewing will probably be about my 12th. But I still know that when it gets to the scenes like the one where the hoods of the police cars start blowing off, I'm going to loose it (Ed O'Neill's face is PRICELESS!). Recommended 110%.

*WARNING* Contains MANY SPOILERS!
Let me start by saying I have a huge respect for Gillian Anderson's incredible talent as a varied and versatile actress - which is why I cannot comprehend her reasons for agreeing to make this film once she saw the script (or lack thereof.)
The premise of the film was, in my opinion, a great idea and there were some genuinely thought-provoking themes in there but it ended up like a collapsed soufflé. It exemplifies why I hate 99% of British cinema. It feels too long, it's tedious, for the most part, and not a lot happens after the first twenty minutes. Just when you think there's a chance of it picking up some speed it disappoints like Paula Radcliffe running a marathon. With little imaginative directing and a minimalist plot, there isn't much to keep the audience from nodding off into their popcorn. As for the script I can only surmise that the writer was trying to save a few trees, with the average scene reading something along the lines of "Alice: F*** OFF! (Adam stares. Adam runs off into woods)(Alice follows) Alice: ADAM! ADAM!" I suspect that, word for word, the actors probably got paid more than Kate Moss did for her Virgin Mobile adverts. What few lines there were didn't have a lot of variation with a frequent use of the f-word that would make Bridget Jones's friend, Shazza, proud. There is little establishment of the main characters before the main sordid event which leaves the audience lacking much sympathy for the characters beyond an automatic 'Oh that's terrible' reaction.
Alice isn't the kind of woman who courts sympathy either. She's got a great job, an expensive London apartment with roof space to die for yet she comes across on screen as conceited, bitter and dissatisfied before her life takes a turn for the worst. After the attack a few layers are peeled back which sort-of explain why she is this way to start with; she grew up with a tough-as-old-boots soldier who thought that teaching her how to shoot his gun was the ultimate expression of love so, instead of following in his footsteps, she ran away to the big city in search of something to make her feel like her life is worth living. Instead she found a group of stereotypical middle-class Toffs who look down on anyone not rich enough to drive a Lexus and the luxuries that come with an integrated security/entertainment system (i.e. becoming Mrs Robinson to a wanna-be Cockney wide-boy electrician) Someone pass me a tissue. The one saving grace of this character is that she is played by Gillian Anderson. In the hands of a lesser actress she would've been intolerably one-dimensional but Ms Anderson actually manages to inject a few fleeting moments of humanity into this otherwise lifeless human being, most notably when she's sincerely apologising for her road rage in a vain attempt to stop her attackers from continuing their assault.
I can't say that Adam fared much better either. Danny Dyer played him well as a fish-out-of-water Jack the Lad but a good performance couldn't save him from both the lack of a script and the total absence of any character background.
This film relied mostly on shock value but the timing was off and it felt far too engineered from beginning to end. As for the shock, the most shocking thing about this film is the unashamed demonstration of how painfully thin Ms Anderson has become; it was almost as unsettling to see as the brutal attack scenes. On a side note, only in a British film would a gang of violent sex attackers take the time to offer each other contraception before continuing to cheer their mates on - talk about stiff-upper-lip taken to the extreme! If this is the kind of film that the National Lottery is donating money to make then I'm not surprised that fewer and fewer people are choosing to spend their pound each week.
Saying that I hated this film is giving it too much credit, I didn't care enough about any of the characters to warrant that strong an emotion. I want that one-and-a-bit hours of my life back, please!