TV - Movies
The Ending Of
The Stephen King adaptation makes it completely clear that room 1408 is not a part of Mike's imagination. Many of the haunting events in 1408 have to do with past visitors or victims of the room, but it eventually gets personal, mixing the real-world horror of trauma and grief with supernatural hauntings.
Room 1408 Was Real
"1408’s” theatrical version ended with Mike's survival and his reunion with Lily. But the movie also had a director's cut ending where Mike die’s in a fire that destroys room 1408 and according to the movie's Wikipedia page there was a third ending which was slightly different from the director’s cut.
Different Endings
The source material from “1408” is from a short story, so the film had to expand and fill out Mike's experience in the room at the Dolphin to fill the length of the movie. Probably the biggest change between the story and the film is the addition of Mike's backstory where he had an estranged wife and daughter.
Movie vs Short Story