This volume collects the short stories into chronological order, from 1923 to 1941. There is quite a variety, though inevitably a few repeated themes emerge. What strikes me in particular is that the first four or five do seem very weak indeed - extremely lightweight and not well put together. The Disappearance of Mr Davenheim in particular seems to borrow ideas rather heavily from Conan Doyle.
But as she gets into her stride, the stories do seem generally more well thought out. In this edition, some of the longer stories are described as "extended versions" of the originals, and these work very well; one can readily transpose them in principle to the superb David Suchet ITV series. For the shorter stories, one imagines that the script writers must have had to dream up padding material to fill the one-hour slots. (The two-hour episodes are generally from full-length novels).
For me, Poirot is most successful when in England: be it in the metropolis, the shires or the seaside. Some of the short stories are set abroad, typically around the Mediterranean and Egypt, and these don't work so well. I suspect that to some extent it is a matter of changing times: whilst the whole world has obviously changed since the 1920s, Poirot's European travels are simply not exotic and the preserve of the upper classes the way they were eighty years ago.
Another thing that strikes me is just how much Christie relies on supposition and confession. Many of Poirot's cases would never stand up in a court of law. But it's a fun parallel universe to visit.
In other news, my copy of Verdigris Deep arrived today. I know what I must do next.