I only watched the first season. It has its high and lows but was mostly mediocre. The cast is really good, the setting equally so. This is not James Bond or Jack Ryan like. It feels much more realistic. But the writing wasn't great and there is little suspense overall. And it is ultimately a story with no one to root for.
[spoiler] You start with the ending so to say. From there on it is a "how did we get there" story. It starts with the whistle blower Thomas Shaw and again it is shown who he is. This becomes a "why does he do it" and a cat and mouse game that, like I said, doesn't deliver much suspense. In the end the ulterior motives for Hector/Shaw stay rather vague. He wanted to change things but why ? Out of love ?
All of this progresses rather slow and when the plot twists finally come they were ridicoulus or not very surprising. Then everything is thrown into the last 15 minutes of the season finale. The big clean-up. [/spoiler]
I read the synopsis for the second and third season and decided I'm not interested. Therefore I dropped the show.