"1408" is one of the best short stories in Stephen King's working career (and he writes a lot and well) but this adaptation is very different from the original and the changes - in my modest opinion - harmed this movie, which could be a masterpiece instead of just another horror film.
In case if you didn't read the story (available in the collection "Everything is eventual") or saw this movie, I do not want to provide any spoilers, so I will not go into the details of the plot - let's just say that this is a tale about a professional "ghost hunter" who stays in places supposedly haunted to debunk the myths and then write about his experiences. After many years of doing that he finally finds one place, a hotel room, which is very different from all the others supposedly "haunted" or "cursed" places he ever saw... Both the story and the film describe his short but eventful stay in room 1408....
As I already said the original story is excellent and EXTREMELY scary! It reminded me of some of H.P. Lovecraft short stories and also of one long passage in one of the earlier Stephen King's book, "Dark Tower III, the Wastelands". In the story the solution is very logical and almost believable (a little hint here - think about how polar bears, who live in the world of icecap, hunt seals, who live in the world of the sea...). The movie is very different from the story, no matter which of the alternate endings you choose, and the result is much less convincing. My advice, if you do not want to have your pleasure spoiled, is to watch the movie first and only THEN read the story - because if you do it in different order, the movie will not taste all that good anymore.
The film is not without merit. John Cusack gives here a very good performance. As it is mostly a "one man show", Samuel Jackson can not really show all his talent, because his appearance is brief. The events which occur in room 1408 are quite creepy and there is at least one VERY scary scene - but sadly, because of the changes in the story made for the needs of this movie, we are also frequently taken out of the room for flashbacks, and they are simply depressing and boring. The character played by Mary McCormack, which doesn't even appear in the original story, is distracting us from the mainline plot rather than adding to it.
So, to conclude, this movie, although honest, is not a real success - if you want to see a really great recent adaptation of Stephen King works, try "The Mist"! That one is REALLY good!