I think Irreversible has much more of a message (and isn't subtle about it at all. It tries to hammer its message into you, like in the early scene in the Rectum). 5x2 is much more about the characters, I quite liked the acting. It also reminded me of Wong Kar Wai. The songs that were used from episode to episode for example. The bittersweetness of the love story. etc.
But yeah, not sure if there is much of a point to 5x2, perhaps it tries to show how love changes over time, how people change, by playing backwards it puts an emphasis on the changes. Love is not (always) forever.
Not sure how I'd rate this movie though. I liked Irreversible more, but both were quite enjoyable (well, if the word enjoyable can be applied to Irreversible). Should watch both again.