I love the manga (it was my first), I love how the characters develop and bounce off each other. The story itself isn't anything groundbreaking though I like it too, but the main thing that attracts me to the series are the characters and their background - Particularly Ishigami. As you've mentioned, it's really good with foreshadowing and building up certain events. Ishigami gets a good amount of this. I feel as though the series really evolves as it goes on, and it's very "simple" in that you can pick it up and read it at length without having it fall flat, in part due to the characters and the style of humour.
Is it a masterpiece? In my opinion, I'd say so, because frankly I don't have much other manga to compare it to and I adore the series. But looking at what it is and comparing it to the other stories out there, it isn't a "masterpiece" to so speak. Plus, I think to an extent that trying to objectively rate something as subjective as this is futile. If you take the series for what it is, and what it's trying to convey, I think it's exceptional and I've recommended it to my friends.
The series isn't some kind of complex epic, it doesn't try to convey much of a deep meaning or anything, but for me that's fine. It's a fun, exciting series full of lovable characters that make me happy. There are legitimate criticisms of the series, and I fully understand that not everyone will like it. Not everyone will like the setting, the series, and there are some who may take an elitist standpoint to look down on it. I accept the reasons people have for disliking it, but for me I think it's the best story I've been immersed in so far - This is why I think it's a masterpiece, because I wholeheartedly enjoy it and it's my favourite story.
