Late to the party but just finished.
Season 1: "B." The balls are in the air, maybe a bit too much soap opera over police show. But very solid.
Season 2: "A." Business picks up when it's revealed there's a mole in the department. (No spoilers!) Eventually they are exposed.
Season 3: "D." So disappointing. Bad story choices, characters doing random things. Creative team really lost their way. Suddenly introduced a slew of new bad guys mid-season! Last couple eps are a train wreck. The actors -- a fine lot -- sleepwalk through the final two shows. What happened?
Overall I'd call it a pretty good show that looked like it was going to be a very good show.