I've friends who hold 5 x 2's director Francois Ozon in very high regard, as if he can do no wrong. This, like some of his others are very realistic, frank, often confrontational and make uncomfortable viewing. He's not my favourite director.
I certainly think it helps if the viewer has had their share of life's relationship 'journeys', too. Not just to know what the dialogue's about but what makes some often rather unlikeable people 'tick' and to shrug off the extremely earthy talk as easily as the characters do.
One of my favourite directors, Ingmar Bergman, could run a 3 hour film about relationships, as in his 'Scenes From A Marriage' and it would still hold interest til the end. 5 x 2 is much more adult, certainly less sweet but is strangely compelling.
As I'm sure you know by now, this film runs in reverse, from divorce settlement right through to first meet. Don't let that worry you, it works perfectly fine and just unfolds almost as naturally as if it were the normal way round. This, my second viewing after many years, on Channel 4, is no easier to watch, however. The acting is first rate, no doubt but that intensity doesn't give one a break and one can become stifled by the film's honesty. However, first love generally being much sweeter than ugly divorce, it does get lighter toward the end, with a rather odd looking swimming into the sunset (as in Westerns) finale.
My head tells me to give it 4 stars, but my heart says otherwise, three. The French songs that Ozon uses for the soundtrack though, are wonderful and get 5!