Eithne’s review published on Letterboxd :
Despite all its post-production related problems, Slender Man could never rise above its juvenile source material. The "character" already doesn't have a well fleshed out backstory or set of rules, like Freddy or Sadako for example, which allows (and requires) the filmmakers to create their own. Which they obviously don't do, instead opting for a series of confounding non-sequitur scares that, ultimately, have no resolution. Parts of this film, many of which are featured in the trailer, were cut out because of a real-life crime, detailed in the amazing documentary Beware the Slenderman, which makes this aimless film even more confounding.
The only part I enjoyed was seeing Javier Botet in a non-prosthetic role and I hope he will appear on screen without FX more in the future.