Like most people, I believe the series is most similar to Kanon, also made by Kyoani and Key. I'm not going to go into detail about how godly these two companies are but I can tell you that Kanon was just incredible.
Story:
As a Key game, I believe I can put great faith in the story. Almost every one of their production starts off looking like a standard harem anime but takes a turn into a deep, mysterious and emotional plotline that is almost sure to make you cry. I haven't played the game, and I don't think I will because I don't want to spoil what happens. I was a bit skeptical at first, but I feel like the story has lots of potential. I wasn't really fond of the first arc because it seemed so unoriginal, but the episodes that followed allowed me to enjoy the story again. Like every other anime Kyoani makes, there are a lot of random things that happen in the plot, but they have always been tied up in the end and I will trust for it to happen with this series as well.
Art:
The art is amazing as always with Kyoani, and I am glad that they have not let me down here. One thing that they do great is expressing the seasons through spectacular backgrounds, I really hope they have wider shots for scenes. The camera is a lot more lively in Clannad, which helps attract the viewer's attention. In terms of art, everything is perfect.
Sound:
The OP in this anime is just great, and one of my favorites. I wish I could say the same about the ending. The first time I heard it it was funny, the second and third time it was cute, and then it just got annoying after that.
Sometimes the voice overs are done in a weird way, but you soon get used to it.
Character:
The characters are always the best part of Key animes. Every character is incredibly cute and likable but has a weird part which seems mysterious until you learn more about. As you watch the series, you gain great insight and understanding of the characters. I guarantee that you will fall in love with one if not all of the characters in this anime. The protagonist is very well portrayed and getting into his mind is very entertaining.
Enjoyment:
Kanon and Air take you on an emotional roller coaster, with parts that make you cry and parts that make you laugh and simply feel great. Clannad shows some signs of this too but it concentrates on the humor a lot more. The jokes are HILARIOUS, there hasn't been one episode where I haven't laughed out loud which is really impressive. I enjoy this anime a lot, even more then Kanon at some parts, but the enjoyment is not consistent.
Overall, I feel that this anime is great, and some parts are unparalleled. People that have watched kanon already might feel that it is not as great, and although some aspects of it surpass kanon, I can't say it is better overall. Still, I recommend that all anime lovers watches this series.
