A great experience I would highly recommend. It's incredible at making you feel the paranoia of the main character and making you question what is real or not. Confusing in just the right way throughout until it all comes together in the end. I really loved the climax, one of the best I've experienced.
Characterwise, it is absolutely carried by the main character. The other characters aren't bad, but aside from Rimi and Nanami, there aren't really any standouts. But Takumi really steals the show and he alone could make Chaos;Head worth it. His voice performance is honestly one of the best I've ever heard.
There's a lot I love about Chaos;Head and not really anything I particularly dislike. The complaints I do have are less things that are wrong and more things that could be made better.