The movie -Kal ho na ho - is definitely entertaining. The performances by Preity Zinta and Saif are very good. Shahrukh, though not bad at all is becoming stereo typed in his facial emotions. The songs are definitely hummable and a reason to watch the movie.
The story line though seems unpractical. I mean would it be practical in real life?? Which girl after knowing that the person whom she has actually loved so much has passed her on to someone else because he is about to die. Would she then be able to love her husband? And the husband(Saif in this case)- would any man practically accept a girl who loves somenone else and that too after that someone else has made such a big sacrifice. I mean we are talking about male ego and pride. True that time is the medicine to everything in life but I am talking about the present.
All this criticism would definitely have erupted and the movie would have been a flop but for the biggie names associated with it.
Anyway since entertainment is what movies are all about nowadays I would rate it 50:50.