For production values (cinematography, directing, acting, casting, etc. - especially the period settings and wardrobes) this is a top notch series. Absolutely top notch. As for the plots, well... they are based on Christie mysteries - which you either like or not. But so loosely that it doesn't really matter. The characters are over-the-top and the whole thing is done with humor in mind. Took me, however, about 4 or 5 episodes to really appreciate this series. Initially, Laurence's coldness, his male model handsomeness, his creaseless suit is a caricature of a male chauvinist cop. Again, part of the humor. But, with Season 2 he really showed a very different side of himself. The acting by all the characters is just superb. But, it does take awhile for this series to grow on you (or did for me). And the mysteries themselves are usually actually pretty good.
