I read the Stephen King short story in anticipation - which was a bad idea. The narrative is used more as a springboard for the further development of the concept. Don't get me wrong - I'm all on board for this expansion of the short story. I love the idea of the totem object that could possibly be a deity or god of some sort and how it follows the brothers into adulthood.
What I was sold was not what I got. I would say it compares to the use of the monkey drum toy for the poster of the film Monkey Shines (1988) by George A. Romero. Which is to say - they have nothing to do with each other - one is used to sell the other - and it's misleading. I blame the trailers. The Monkey - in terms of a film - is closer to a real bloody version of The Cable Guy (1996) or Very Bad Things (1998). If you go into the film expecting the tense horror/suspense of Longlegs, you'll end up a sad monkey.
As I walked away from the film I think I would have really enjoyed it if it was marketed more as a dark comedy (and sure some of this blame is on me as a viewer to do my research). As it stands - if you've seen the trailers you've already seen 90% of the suspense in the film. What you haven't seen is the absolute mayhem and chaos that surrounds those situations. I think the first 10 minutes of the film tells you exactly what it is - how the humor works - and how it plays out through the monkey.
If you're on board with the opening - it's a perfect roller coaster. My problem was that I thought I was on a rollercoaster and instead ended up tripping on a bunch of banana peels. Regardless, I think the film is worth viewing (and to continue using animal innuendos) you might find it a hoot! But in the end, the film is just monkeying around with the audience.