Between 1981 and 1996, the Star Wars Original Trilogy was adapted into a series of radio dramas, the first two of which aired on National Public Radio. Brian Daley (author of the Han Solo Adventures) wrote the scripts and several of the film's actors reprised their roles, including Anthony Daniels (C-3PO). In addition to retelling the story of the films, the radio dramas included much background and character information, and thus were an early part of the Star Wars Expanded Universe.
Episode IV: A New Hope (1981)
The radio adaptation of A New Hope is six and a half hours long, broken up into 13 half-hour episodes. While this sounds agonizingly slow (and sometimes it is), the radio drama is structured as a serial, each episode as its own story, and most of the extra running time comes from additional material that explores and expands the characters. (The story of the film itself doesn't even start until the middle of Episode 3.)
Overall, the additions are a mixed bag. None of them are necessary to the story, although many, like the opening episodes exploring Luke's relationship with his friends and Leia's role in the Rebellion, are very interesting. Only one is particularly terrible: the scene where Vader tortures Leia, which ends merely with a menacing closed door in the film, goes on for so long as to verge on parody, and is probably the most uncomfortably awkward scene in all of Star Wars save for Itchy's sex fantasy in the Holiday Special.
The story also runs into the same problem as Alan Dean Foster's novelization: a weak and non-threatening Darth Vader, this time because his voice sounds smarmy and bored rather than strong and demanding. The rest of the cast is good -- particularly Mark Hamill reprising his role as Luke Skywalker -- but that only makes Vader stick out even more.
Episode V: The Empire Strikes Back (1983)
The radio adaptation of The Empire Strikes Back is only ten episodes. In some places the pacing feels tighter as a result, but in other places there's just less additional material. The added material is again rather up and down, with the down parts surrounding the budding relationship between Han Solo and Princess Leia. On the one hand, Han's character development from rogue to respectable is much more explicit in both this radio drama and the previous one; but on the other hand, he often comes across as too much of a jerk for the audience to sympathize with him.
The duel between Darth Vader and Luke in the final episode makes up for most of this radio drama's flaws, including the continued weak portrayal of Vader. The scene where Vader cuts off Luke's hand feels so much more emotionally charged than the same scene in the film. Vader's plea to his son to join the dark side feels impassioned and almost vulnerable -- the only place in The Empire Strikes Back where such a portrayal of Vader really seems to work.
Episode VI: Return of the Jedi (1996)
The Return of the Jedi radio adaptation is the odd man out in this trilogy. After sitting on the back burner for over a decade due to budget problems at NPR, it was eventually produced by HighBridge Audio, the company that distributed recordings of the NPR radio dramatizations. At only six episodes, it feels less like a serial and more like a longer version of the movie.
The main difference between Return of the Jedi and the earlier radio dramas, however, was the presence of the Expanded Universe. Daley incorporated existing Expanded Universe material into the script, including Mara Jade in disguise at Jabba's palace and references to the events in Shadows of the Empire. As a result, this radio drama fits more seamlessly with the rest of the EU than the earlier Original Trilogy adaptations.
Much of Return of the Jedi doesn't lend itself to audio adaptation as well as the previous films, so there are a few awkward spots, particularly in Jabba's palace. One place where the need for extra dialogue to replace visual cues really works, however, is the final scene of Luke burning Vader's body, which feels a lot more subtle and poignant than either the original film or the Special Edition.
The Star Wars Radio Drama Trilogy
Overall, the Star Wars radio dramas are worth a listen more for supplemental material about the characters and universe than as adaptations of the films. The A New Hope adaptation in particular goes into a lot more depth about the characters than is possible in a movie, a novelization, or anything not in a serial format, really.
Most of the parts I disliked relate to either pacing or voice casting; but those issues just come down to a combination of opinion and how much you like radio as a medium. If you have the same dislikes, reading the published radio drama scripts rather than listening to the productions themselves solves both problems. If you listen to nothing else, however, listen to the final episode of The Empire Strikes Back.