As the next to the next to the next to the last Poirot, there is only mediocrity in this 35th novel entry into the fussy Belgian detective's canon. Christie is clearly recycling plots - this one has elements of Taken at the Flood and Lord Edgware Dies - while making rather unsuccessful attempts to remain relevant into the swinging sixties by dropping awkward references to druggies, artistes and the Beatles.
The plot itself was contrived and even more absurd than normal. Since I know that OB will be reading this one, I'll save my comments about its utter lack of realism until she posts her review. It wasn't great as a whodunnit (and I figured it out pretty early on in the story), but it would truly suck if there wasn't even the payoff of a solution to the mystery at the end.
Conclusion: Not as bad as The Big Four, but no where near as good as Christie at her best. Skip it, unless you're on a quest to complete the Poirot novels.