Tokyo Ghoul is a manga adaptation that follow a well-trodden but disappointing arc: beginning with an unusual premise and descending into much more generic shounen super-powered fighting.
The setting is Tokyo openly infested by ghouls, with a special police force assigned to combat them. The main character becomes a ghoul against his will, discovers a new craving for flesh (and no other food), and finds himself in the strange combative society of ghouls. As the series progresses, there are more mysterious characters with special powers and more fighting, until an opaque three-way battle that closes out the season. Be warned, the series is gory and contains scenes of torture and viscera-consumption.
The second season almost seems like a different anime, following the police and making the previous protagonist an unpredictable character who pops up now and again. It’s far more concentrated on the fighting, and it’s difficult to understand what’s going on at all.
The manga appears to have a strong following, and reading online descriptions it appears that significant storyline-relevant material was omitted from the second season. Nevertheless while the animation (particularly the one-eyed ghoul at the end) and atmosphere are thick, too little story content is depicted.
Like Fate Zero, Knights of Sidonia, and Durarara 2, Tokyo Ghoul is scheduled to continue in another cour, with the last season ending rather unsatisfyingly.