At first, I thought it was interesting, but not special. I kept watching and by the end of season 1, I was convinced this was an excellent show. There are 3 seasons, but by the end of the 3rd season, the series wraps itself up quite excellently. Do I want there to be more? Absolutely! But at least there was a satisfying conclusion.
So what's good about it? First of all, the acting is great. The writing is excellent. The idea and story is original. Furthermore, it's a story about time travel that has its own rules and is internally consistent, which is something that a lot of time travel depictions do not have.