And my conclusion is that it's pretty fair. The are a couple of important clues in the early chapters right up to the discovery of Roger Ackroyd's body; we are then misdirected by the seething discontent and deceit of the Ackroyd household, which presents several sub-plots which have to be resolved one by one, until we are left only with the original question. Unlike And Then There Were None and Murder On The Orient Express, this is a pretty ordinary crime, done for ordinary reasons, extraordinary only in the way the solution is revealed, and I think all the stronger for it.
Roger Ackroyd is not poisoned, but there has been a poisoning shortly before the story starts, and it ends with another. In fact I believe that poison is a relatively rare method of murder, both in real life and in fiction (for Sherlock Holmes, whose adventures are often, like Ackroyd, narrated by a doctor, I can remember only A Study in Scarlet; for Ian Rankin, I can't remember any at all). Unfortunately we have killing and death constantly with us; Agatha Christie's genius is to isolate these fears by using bizarre methods and nested circles of isolation where these events take place - the secluded village, containing Ackroyd's house which is in it yet distanced from it, in turn containing Ackroyd's study where he must not be disturbed.
I'm watching Poirot with interest for signs of Belgicity. (Saying septante instead of soixante-dix, that kind of thing.) Nothing yet to indicate that he is other than an eccentric Frenchman, alas. The least plausible parts of the novel are the rapidity with which he takes the narrator and his nosy sister into his confidence, and his decision (as in Murder On The Orient Express) to thwart state enforcement of justice in favour of his own interpretation of natural justice. Perhaps the second of these is a case of implementing the famous Belgian saying, on s'arrange, but I don't really think so.
Anyway, I'm enjoying these much more than Lovejoy. The only problem is that murder mysteries are not great as insomnia reading...