To catch you up a bit if you've missed out: The other adaptations of this season were Three Act Tragedy, The Clocks, and Hallowe'en Party (all of which have aired in different places throughout the world but none have aired in a single country yet as far as I know). Three Act Tragedy's adaptation was actually very faithful, and I would in fact recommend it over the original novel for newcomers. For MotOE, I would push for you to read the novel just because I can't say what they will change in this, but--for the most part--the adaptations are phenomenal if only because David Suchet is spellbinding as Poirot.
This season overall has been quite good, compared to some of the other recent ones which were usually just pretty decent. With the exception of Appointment with Death, of course, which was a travesty. Hallowe'en Party was one of the weaker Christie novels, but the adaptation set out not to spend much time on the duller parts and gives it an atmosphere that Christie really didn't convey very well in the original (even though it was implied). The Clocks is one of my favorite of the Poirot novels, and I was skeptical that it could be made into a 2-hour drama without some sacrifices. Fortunately, I was very pleased.
Of course, anyone that has ever seen the Suchet series has been looking forward to Murder on the Orient Express after watching their first episode--it's the most famous Poirot story and arguably the best. For the perennial Poirot works, they don't have too great of a track record and instead seem to excel on the lesser known stories. The Murder of Roger Ackroyd, for example, had the worst adaptation in the series outside of Appointment with Death.
I'm curious how they will make it work, and for those of you that have read the novel and have access to PBS, I highly recommend that you give it a watch. For anyone who hasn't, well, it's up to you on whether you want to watch it or not. I have faith that Suchet & company will deliver, and you should too!

After this season, I highly expect them to just do the four remaining novels (Elephants Can Remember, The Big Four, Dead Man's Folly, and of course Curtain) and forget about the others (namely Black Coffee and The Labours of Hercules). Those saying they need to waste time on the remaining short story, that play, and that final collection are mental, to say the least.

Some other notes:
- PBS will be showing the Third Girl adaptation on July 18th, and Appointment with Death the week after. I actually recommend seeing Third Girl just for a pretty decent adaptation (which deviates pretty greatly at times, but casual readers wouldn't mind the changes I imagine), but only watch Appointment with Death if you're aiming to make the title reality. It really is that dreadful, sadly.

- A Suchet documentary of him traveling on the Orient Express that aired on ITV in the UK last year is now airing regularly on PBS. It's actually a very charming travel log, and if you have seen Suchet as Poirot but never as himself, prepare to be blown away. You'll have far greater respect for his acting just by seeing his normal appearance and hearing his normal voice.
TL;DR VERSION!: One of the greatest mysteries of all time has been adapted with the greatest Poirot to portray him on screen and will be premiering on PBS at 9:00 PM EST on July 11th!