Its a personal diary, so its not written in a way thats meant to be read easily. Theres a lot of minutia about plays which he saw which isn't too interesting. However, his descriptions of existential angst are pretty cool. Kafka was a depressive, but this makes him seem more depressed than he probably was due to the fact that diaries are just generally filled with people dumping their anxieties. Problems with talking to people, esp women, family problems, etc.
Also great are his descriptions of the everyday behavior of people around him, which was obvious practice for his books. I particularly like his descriptions of women.
Also lots of jew-talk in here. References to the activities of contemporary Zionist groups and Talmudic scriptures etc.
Stay away unless you love Kafka and/or diaries.