Interesting mystery....though maybe a bit too complicated. 

I like Bobby and Frankie and found their banter and friendship to be pretty cute. I think that will probably be the best part of the movie adaptation (unless they messed it up!)

The villain's "you meddling kids" speech had me both tickled, as a fan of that kind of things from Scooby-Doo and the like....but also rolling my eyes because.....I also find that predictable writing. BUT because this is Agatha Christie....and she's from an older age of mysteries....
maybe that trope wasn't so played out back in the day. So I'll give it a little excuse.