This series started out great. Season one has great scripts, good characters, compelling plots and beautiful cinematography.
Season two rolled around and the production staff must have changed hands. Instead of nice, tight, two-part stories, we got a long, rather boring story arc that was confusing and consisted of catching a bad guy portrayed by an actor whose sole ability was to glare malevolently through smoke-hued glasses. The cinematography began to resemble Hollywood's, there were too many fight and shoot 'em up scenes.
Season three rolled around and is barely watchable. I don't even think I'll finish it. I think everyone quit giving a damn, including the actors. The plot is like caramel apple rolling down a dirty street, collecting whatever bits of debris lie in its path.
So, watch season one. It's great and beats the pants off anything out of Hollywood in the last 15 years. Watch season two, but be forewarned. Watch season three if you're a masochist or are stuck in a hospital bed with nothing better to do.
PS -- The subtitling (English) is excellent, except for the amusing omission of a certain swear word...