I hate the fact that people think just because it is a light hearted romance that isn't super serious that it is supposed to be a comedy. I don't think it was meant as a comedy but even if it was it is far better than anything Will Ferrell has ever been in.
I didn't take it super serious as to how possible the plot line is, and I've heard stranger real life stories. I didn't take it as a comedy either. I took it as a romance among non-dramatic people. If you let the movie be vs trying to force it into some stereotypical genre you'll probably enjoy it. I will say I didn't think the ending was the best or true to the characters, but overall a fun movie.