Of course, much is lost in the translation of "The Good Soldier" from page to screen, chiefly the narrative and temporal complexities of the novel. However, the television adapatation is successful in its use of time shifts, and, like Ford's work, the film is impressionistic, often showing us a scene twice to give the viewer two different impressions.
However, we lose Dowell's process of writing itself and his relation with the reader. In the film, Dowell is mainly absent from the last third of the story because it is told from Leonora's perspective, whereas in the novel Dowell's voice is dominant throughout, even when he is recounting what has only been told him.
Of course, any book - especially a modernist novel - loses a good deal of its complexity in the transition to film, and "The Good Soldier" is on the whole a successful adaptation of the novel, much better than the adpatations of other modernist novels I've seen, like "Ulysses" and "Mrs. Dalloway." The film was shot on location in Nauheim and elsewhere in Germany, which shows the care this production takes with Ford's work.
The four principals are all excellent in their roles, particularly Jeremy Brett and Susan Fleetwood as the Ashburnhams. The score was excellent as well. The production also keeps Dowell's voiceovers to a minimum, which may be something of a loss but it is welcome in a televison adpatation, which thankfully shows us a great deal of the action instead of telling.
Overall, then, I found this adaptation of "The Good Soldier" to be one of the more successful installments of "Masterpiece Theater" and probably one of the best film made from a modernist novel that I have seen. While we lose the complexities of Dowell's narration and his potential unreliabilty, the film utilizes the time-shifts and impressionistic technique of Ford's novel, and there are subtle hints that Dowell may know more than he lets on. Thus, the film's success stems from its faithfulness to the aesthetic spirit of Ford's novel.
Caveat: The film will make little sense at first to those who have not read the novel, which is required reading for anyone who wants to understand the nuances of the story.