Season 1 was very good. Story line is well written and the characters are well developed. You really get to know these people and what drives them. Season 2 was also very decent, only slightly below season 2. Season 3 started very promising (only the first episode), but from then on, it went downwards and never came up again. Season 3 introduces new important character, which they shouldn't have.
Season 4 is even worse than season 3. I never stopped watching it, because I always hoped it would get better again. But it never did, until it got cancelled.
My rating is only for seasons 1 & 2.