It is rare that you see a film adaption which is so similar to the novel it is based on. I'll say that for Secret Window- it is very true to the book. This is a good thing- it is really frustrating when you are a fan of a novel and then it is completely changed when a film is made of it.
Johnny Depp, with his usual quirkiness, is perfectly cast as Mort Rainey. If another actor had had the part and not carried it off, it would have been an awful film. Mort's character is well defined and likeable. Although he is very much the central character and there are not that many other characters, and most of the film takes place in his house, the story does not get boring, drag, or feel unvaried. There is enough suspense and scary bits in the movie to keep you on the edge of your seat and wanting to know what happens to Mort in the end.
There are elements to the storyline that perhaps lack originality (for example, Mort conveniently is staying alone in a house in the middle of nowhere), although if you like soft horror/psychological drama films then you will enjoy this. It is not revolutionary- it is simply an entertaining film.
In response to what the other reviewer has written, I think a 12 rating was OK for this movie. It is not as frightening as some other cert 12 films I have watched (The Others, for example, scared me to death the first time I watched it. And I'm 18).
The reason I gave the film 4 stars instead of 5, was because of the twist in the ending. I think the concept of the twist was a good idea, although the way it was done I was not entirely happy with. I felt it was a little too over-the-top in the way it was portayed and perhaps a little left to the imagination would have been better. (This is hard to describe without giving away what happens- but if you watch it you'll see what I mean).
Nevertheless, the rest of the film is great, I will be buying it when it is released, and I would recommend it, especially to other fans of Johnny Depp (who is gorgeous in this movie, incidentally). If you liked Secret Window then I also recommend you read the novella by Stephen King. It is included in "Four Past Midnight".