Orson Welles' so called "personal favourite film", based on the novel by Franz Kafka does remain more or less consistent with the original written work, though Welles has changed a few things to amplify effect or to introduce characters.
The poor quality of the reel (the film was nearly lost), also make it that some lines go unheard and make characters difficult to identify. The "mise en scene" also adds to this confusion, with Josef K. walking through numerous strange rooms, corridors and halls.
Still though, the film can help visualize certain elements from the book that might be difficult to imagine otherwise. Also, it is important to remember that Kafka's work was unfinished and unassembled, and Welles has attempted to reflect this non-linearity within the film.
All in all, I do recommend you see this film if you have read or plan on reading the original work, but you might be very confused by the storyline without this base knowledge.