Crisp storytelling, perfect tonality and a damaged hero struggling to find his wife & daughter's killer to have his revenge. The first half of the series up until Jane (the protagonist) catches up with the serial killer is intriguing. The giveaways towards the end of episodes that help catch killers is also amazing. Minor habits of characters and small clues, beautifully woven in episodes that reveal the killers most of the times make it very Sherlockuesque.
Simon Baker's portrayal of the character is amazing.
The series falls loose however, after season 5, with lack of a driving point for the hero to pursue killers, leading to show being drab, dull and uninspiring at most times. The absence of a recurring theme, after Season 5, also abates the appeal of the the show.
The series starts with a punch in the first 3 seasons, stabilizes up until 5 and then drops after.