I'm going to keep it short despite having a lot of feelings about this anime.
ending was ass but I won't deny that this anime is going to be remembered forever as an iconic piece of work.
It was an immensely ambitious and unique story that would've been very hard to execute well had it been written and created by someone else other than isayama, so I commend his brain. It's just... I feel this series justifies just how important endings are for stories. Characterization and character development was butchered in a span of an hour. Plot holes and unanswered questions were also prevalent. I wouldn't feel so strongly about these flaws had the story been mediocre, but everything in totality was pretty great.
Also, the animation was so wonky near the end of the series (i personally didn't care but it can get distracting sometimes). Actions scenes were still great.
If you don't care about endings for stories, watch the whole thing.