Script versus short story
Christie's original story is particularly short, and it is certainly a testament to Horowitz's skill that his considerable additions and changes largely do seem to work. Most importantly, perhaps, he adds an entire back story of a coup d'etat during a general strike in Buenos Aires, witnessed by Poirot and the other dinner guests (Poirot is almost faced by a gun squad at one point!). This seems to be inspired by the real-life 1930s events in the country. He then goes on to link this back story to the one in the original short story, concerned with Barton Russel's speculative investments together with Stephen Carter (in the adaptation, they engage in some shady business dealings with the Argentinean military on oil fields). Also, as I have already implied, Poirot is present at both dinner parties in two near-identical Jardin de Cygnes restaurants (one in Buenos Aires, one in London). Horowitz also adds a section of the story set shortly before the second dinner, in which Poirot interviews all the dinner guests. Some other less significant changes are made, too, like expanding the potential love affair between Lola Valdez and Barton, and adding a scene in which Hastings visits the Wetherby solicitor in Reepham. Finally, and rather cleverly, Horowitz adds a denouement in which Pauline dresses up as a waitress after her "death" and serves the other guests coffee while they listen to Poirot. That is a nice way to make the "one never looks at a servant" clue somewhat more believable, since all the "suspects" are easily misled. All in all, then, I'm inclined to say that Horowitz's adaptation is a success, even if his choice to include a South American visit for Poirot does seem like an out-of-character thing for him to do.
Directing, production design, locations, soundtrack
Barber-Fleming's direction is highly effective, particularly in those flashback scenes to Buenos Aires. The production design here is certainly impressive, with sets ranging from abandoned hotels to cabaret theatres in London. The episode was presumably shot largely on location in Spain (together with 'The Adventure of the Egyptian Tomb'), with most interior scenes shot at Twickenham Film Studios. Gunning's soundtrack is highly effective for the episode, but it has sadly not been released. Particularly memorable is the song, 'I've Forgotten You', based on Christie's lyrics (it was presumably orchestrated by Neil Richardson (who did most of the orchestrations for the first six series).
Characters and actors
Hastings and Miss Lemon are added to the story, admittedly in very minor appearances, but their presence does actually work quite well. Of the guest actors, no one really stands out, but Geraldine Sommerville (known to many as Harry Potter's mother) is memorable as a "murder victim" / motive.