Just got finished watching this film. As far as horror-thriller movies go, it did its job; the cinematography was top notch for a horror film without having to depend on derivative jump scares, the pacing was pretty good, the plot straightforward enough and the theme had potential. Not so much the whole "AI is going to kill us" (which completely lacked in subtlety of any sort) but on the importance of the bond between parents (or guardians in this case) and children and how technology is slowly eroding that bond. I say potential because, outside of a few key but unnecessarily saccharine (and barely believable) moments, this theme was woefully unexplored.
But what really ruined it for me is how far they pushed the limits of "suspension of disbelief". A robot doll kills somebody to defend her child playmate? Okay, I'll buy that. A bipedal robot doll who gets down on all fours (because that's a logically efficient form of mobility I guess?) to chase a kid down, lecture him about manners and rip his ear off before pushing him off a cliff because... yeah, well, basically it only gets worse from there, with scenes between M3gan and other human characters that make absolutely no sense ("let me slowly explain my whole diabolical plan to you before killing you" because sure, why wouldn't a robot with a logical objective do that?) but I don't want to spoil the movie for anyone.
So basically if you're very good at suspending disbelief, can ignore how trite the Terminator-esque message is and wanna see what a sci-fi Chucky would look like, check this movie out. Otherwise, steer clear.