One thing needs to be said at the beginning: the mini-series is not for everyone. It's a kind of production one either loves or hates, and even though I'm in the former group, I completely understand people from the latter.
The thing that divides the viewers is the way Poirot was portrayed in the series, which is very different from what we are used to. Here we get a much darker and grittier version of this beloved character, and the whole world of the show is much more brutal as well. This is not the cozy, quite light story shown in the classic vesion with David Suchet.
I really enjoyed this new approach to the classic character. Otherwise it would be the same story with different actors, and I don't see the point in doing that, especially considering how good the Suchet version is.
The actors fit perfectly into this new "dark Poirot". John Malkovich is brilliant as the shunned, half-forgotten detective dealing with past trauma, Eamon Farren is perfect as the confused Cust, I even liked Rupert Grint even though I wasn't sure about him at the beginning, The sets and cinematography help build the dark, gritty atmosphere. It feels quite heavy and oppressive, which I really liked.
To sum up, the series is not for everyone. It's definitely not for the purists, who want the adaptations to stay truthful to the novels. It's not for people looking for a cozy show to watch for pure entertainment. But for people like me, who enjoy new tales about classic characters and who love dark and gritty stories, it is perfect.