I have to say that I agree with most of the comments of the previous reviewer. It is one of the most emotional Bollywood films I've ever seen. I wept like a small child and even felt compelled to write this review, my first.
Kal Ho Naa Ho is really Shah Rukh at his finest. I notice that Kareena Kapoor was originally offered the role of Naina and I must say that I am glad she asked for too much money because Preity Zinta completely changed my impression of her with this film. She displayed much acting talent in a complex role and is no longer "the annoying girl from the Liril ads" to me.
This is very much like an Indian remake of "Autumn in New York", though, and despite knowing this I was not prepared for the conclusion. If you just like singing, dancing and happy Bollywood I'm not sure this one is for you. But, if you are looking for a plot and character-driven Indian film, I completely recommend this one. Get a nice warm blanket and a box of tissues ready though.