I'm not exagerating giving a 10/10 in almost every aspect of this manga.
Art is not really the forte of Isayama, even thought his style evolved over the years.
But when we talk about story and characters, Shingeki no Kyojin has a really rich universe, that transforms completely over its progression.

The final arc is completely the opposite of the feeling you get reading the first arc. You will begin loving/hating Eren Yaegar and will end hating/loving him.

It's totally worth it to keep reading, even if the begining feels a little like a cliché zombie manga. Every story note is gray, there's no evil, good or condemnable choice made by the characters, like in any other shonen, which makes the story more mature and grim.

If you're feeling a little hard to understand the action scenes, the anime adaptation is excellent.