Note: this review is more of a summary of how it works as an adaptation of the visual novel that's its source material. May not be very helpful to someone who hasn't played the visual novel.
Creating believable horror usually exploits the fact that we fear the unknown. Therefore works of this genre mostly seem to be located either in abandoned, isolated locations or in settings with a lot of superstitious beliefs. The visual novel Chaos;Head, on the other hand, was delightful due to being set in present-day Tokyo, of all places, and still managing to create a threatening atmosphere despite the fact the protagonist could just Google anything he didn't understand. The fact that he's among the most messed up individuals I've seen didn't hurt in this regard, either.
Which leads me to the biggest problems with the Chaos;Head anime. It was just plain too short. There was too much to compress into 12 episodes, which led to rather fast-paced storytelling without leaving any room for the smaller events and quieter moments that filled the game and slowly got the player used to the characters. In the anime they didn't really get enough time to shine before dramatic revelations happened... but if you haven't had the time to get to know and love the characters, they lose their effectiveness.
Then there's the fact that there's a lot of infodumping here. And I'm still not sure if it makes sense to someone who didn't play the VN first. At the very least they don't make it very easy.
Also, Takumi's inner monologue was mostly cut, which hurt the atmosphere considerably. A big part of the charm in the VN was knowing the main character wasn't exactly the model picture of mental health, which made pondering what he saw was even real rather interesting. Well, some of that is bound to happen when switching from first-person storytelling to an anime, but I think it went somewhat overboard.
Setting aside these points, though, I did enjoy it. Was rather nice to see a whole lot of scenes that were only described in the VN get animated and shown properly.Not to mention we see most of the girls actually getting to battle, here.Some blanks were even filled in. To a person whose first contact into Chaos Head is through this anime, though, it does probably seem pretty chaotic and rushed. So, basically... my advice would be to play first, watch this afterwards.