Definitely recommended, just bare in mind this could have been a solid 10 film. Unfortunate.

Rating: 7/10 (4 January 2019)

I'd like to start off by saying that most of what this review is about to be is a statement of the obvious. Therefore, it starts with me stating the obvious and recommending that while you do take what myself and others have to say to your attention (as it's obviously of some interest to you or wouldn't have been reading this now) you shouldn't let any review, be it negative or positive, deter you from giving Split a fair chance. I'm about to dish out some criticism which might even exceed the compliments, and I still urge you to watch it. It's worth that if not more.
So first of all, like previous titles (namely 6 Souls / Shelter, which is pretty decent by its own right and also worth watching) Split not only fails in offering an even remotely accurate account of Dissociative Identity Disorder (NOT the same as Split Personality, by the way) - but obviously doesn't even try. Not to mention it's another one of these films in which the actual fear factor staying with the viewers weeks after is their ignorance of mental illnesses and their fear of the unfamiliar sadly translated to stigma. I am far from being a "woke" activist, but come on, at least try to get with the times, or at least not to make things worse for those battling psychological disorders.
Second, directly related to the aforementioned is the blunt use of pseudo-science (and I use the term loosely as there is hardly anything scientific about the portrayal of DID in the film). Using ignorance in order to present disinformation that ends up being perceived as disturbing and even terrifying only on account of said ignorance is not "cheating", it's simply a little cheap. And speaking of cheap - how could I ignore the plot twist and ending? Imagine somebody is asked to prepare a meat dinner. The guests arrive, the aroma surrounding the dining area is exquisite, and the host presents their creation - hamburgers. The ending of Split is equivalent to that scenario, in which the agreed upon rules have not been broken per se - only taken too literally and used to exploit an obvious loophole of which the decent thing would have been to ignore or at least overlook. The shift in definitions in order to fake a twist and the 10 seconds guest appearance in order to justify it are just like a hamburger diner - undoubtedly satisfying, even good, but cheap.
Which brings me to the acting. If anyone wasn't yet aware that antagonist James McAvoy is a superb actor, for instance if they missed on out on all the newer X-Men films, Split is absolutely a good way to become informed. He's professional, authentic and by all means a delight and the best feature of the film. Which is exactly the problem. Acting the role of different personalities (and operatively different people) residing inside the mind (and to all relevant extents body) of the same individual is really having to act the role of different characters, at times one after the other or even having a conversation with each other. McAvoy does that exquisitely, but simply not enough so. His impressive acting abilities only serve to emphasize this shortcoming, as it feels he's doing everything right yet still getting it wrong. I personally felt very aware that I was watching an act and completely lost all hopes of immersion (perhaps the most important component of a worthy viewing experience when it comes to Psychological Thrillers accidentally tagged by some as Horror). It didn't feel like different personalities in certain times, it felt like an actor failing to adopt the proper minute variations of body language, facial expressions and speech intonation. Very frustrating considering that after watching McAvoy's profound skills I simply feel this mission cannot be accomplished (I would be overjoyed to be proven wrong).
The rest of the cast, while not nearly as impressive as McAvoy, have also performed in a level worthy of commending, and still wanting. The incredible Anna Taylor-Joy who had been excellent in The VVitch was great in Split, delivering a very relatable portray of a teenager dealing with severe childhood trauma. And yet, while most "teenage girls" stereotypic characters tend to be stereotypically and quite annoyingly overdramatic - Casey is under-dramatic to a fault. I do believe direction to blame, but still, you can't show her obviously terrified and yet so preposterously calm. As for Betty Buckley as the conflicted psychiatrist - again, impressive, but not nearly charismatic enough to confidingly deliver the character of an experienced professional. Again, I can only blame direction for these two cases of botched acting, as the cast seemed more than capable to deliver a better performance by far.
Having said all that - none of this criticism is experienced over the surface while watching. Split is suspenseful, entertaining and smart and I personally had a great time watching it, as I would have a great time feasting on that hamburger dinner. The reason that I unfortunately had so much criticism was that Split could have, should have and would have been a masterpiece with better direction and just a little more effort by Master M. Night. It makes me personally sad to see such wasted potential and such a mediocre utilization of God given talent. And yet, I've enjoyed watching it and I would wholeheartedly recommend it to just about anyone.
This has unintentionally been my longest review as of yet. If you've read this far, thank you!