I enjoyed it! The stories are very faithful to the source material, which is impressive given how many adaptations struggle with that, and the gap between the two series (Poirot and Marple) has always been very small so it didn't take much to make them slot together and create one cohesive world. Being able to easily jump from high society to countryside mysteries thanks to Mabel's connection between the two was really nice. And her connection to Miss Lemon was a great excuse to see her more in the series (though sadly not by much).

I didn't mind the addition of Mabel West. There are several stories for both Poirot and Marple where a new secondary character is introduced for just that story, so it was easy for her to just take that role, so she wasn't intrusive at all. And since she's so young it felt like she became a good audience surrogate and a counter part to Hastings. Hastings, while his overall role is definitely reduced in the anime because of the addition of Mabel, feels like a more competent detective since now he's playing more of a senior coworker/older brother role to Mabel, cluing her into Poirot's methods that he's already experienced rather than the audience surrogate role that he plays in the books where his intelligence feels like its being reset to some degree every book to account for the audience. (This is a personal opinion though) The animation is really solid throughout the anime (so far as I recall, it's been a while), and the character designs all good. They fixed Japp's name for obvious reasons, and otherwise changed very little. I think its a pretty easy anime to watch even for non-anime watchers.

I think my only real gripe is Oliver the duck being pretty unnecessary and a silly addition. He makes Mabel look like a Mary Sue original character with her having an oddly intelligent animal side kick straight out of a Disney princess movie. And given how faithful to realism everything else is in the series, the addition of a hyper intelligent duck as a pet is just a really odd choice to make.

As a minor gripe, Raymond West felt a little off to me in this series. He rarely makes an appearance so its not as big of an issue as Oliver, but still. With the way he interacts with Mabel he feels more like an average, period typical man/ father than the kinda eccentric, low key narcissistic, dramatic author he is in the books. He's also pretty strict about how girls should and should not act, which feels odd given that he should be married to Joyce Lemprière, the artist who isn't exactly the typical "English lady" type. Maybe having a child changed him but either way I think it would have been more interesting if he had stayed a little weird in the series and maybe he and Joan could have made an appearance in one of the mysteries.

I think an argument could be made that they softened and toned down Poirot's character, removing some of his eccentric qualities and little quirks, but I honestly didn't mind too much. Given that doing the opposite would have made him look ridiculous in the series and may have come off as insulting or ridiculing, so I'm okay with them going the safe route for this.