It’s an okay film, nothing I feel compelled to revisit, and I don’t have a strong recommendation for it. I like Joey King’s work, but even her performance couldn’t really pull the movie out of its problems. The production quality is noticeable; the special effects are well-done, and there are several scenes meant to create suspense, yet I simply didn't connect with it. The script is the main issue. There are a handful of underdeveloped characters who have just enough distinguishing traits to tell them apart, but they failed to engage me. The movie tries to focus on the gradual descent into madness that Slender Man supposedly causes, but it doesn’t do a good job establishing that tension or showing the characters' mental deterioration—something which could have allowed talented performers to improve things. Instead, there are a lot of instances where events occur without being shown, or abrupt transitions skip over time, and we just see a character suddenly appear bewildered rather than witnessing their experience firsthand. Perhaps it’s not the writing itself, but maybe the editing was flawed? It felt like the editor might have been thinking, “Why are these scenes so long? This is tedious! *cut* Hopefully they understand.” With all the other choices available, I don't believe this film is worth your time. You could probably watch just the final act and get about as much out of it as you would from watching the whole thing.

Source on TMDB